user618: There are a lot of websites to train your tactics, but I can't find any for opening repertoire. I am looking for something similar to 'Chess Position Trainer' but online, where I can import a pgn file with my repertoire, indicate that I want to train it as Black and track my progress. Does such a software exist?
UPDATE/NOTE: being able to train against my own reperoire, and rather than the best tree/stats move is vital for me.

user8309: Chess.com has a puzzle mode in its chess diagram editor.  Perhaps you could create puzzles for each line in your repertoire, and save them to a group forum.

user34193: I created a web application to help build and memorize chess repertoires.
It is completely free and you can use it from any device.
The app is still in development so everything is far from perfect, but I still wanted to share it because it can be useful to you as the key features (build & train) are functional.
I am interested in having feedback to continue in the right direction.
I am not a company and I am alone in managing this project, I apologize in advance for any problem you may encounter while using it, I will try to do my best.
Feel free to try here https://chess-repertoire-companion.com
Any kind of help around this project would be greatly appreciated! 
Cheers

user3456: Actually, I am the author of a website tool that answers the question to some extent. You can load your own PGN file and train yourself against it as a one-time exercise with no progress tracking. You can replay the exercise by revisiting the dynamic link that encodes all the chess data.
This sample link shows a demo. The task is to play White against the Hanham concept in the Philidor defense, which is considered refuted by opening theory.
I wrote the tool for myself and never found the time to provide a user guide but it should work for everyone once you learn how to operate it. I can help with any questions.

user15847: ChessX might be something you are looking for, it has a training mode that uses a PGN file that you give to it.

user3456: There is a new offering from ChessTempo.com!
https://beta.chesstempo.com/opening-training

Welcome to the Chesstempo opening training tool. The opening trainer
  allows you create and manage your opening repertoire, and then train
  against your repertoire using spaced repetition to optimise your
  learning. The opening training also integrates with our play chess
  online feature, allowing you to get feedback after each game on where
  you deviated from your opening repertoire, or extend your repertoire
  to account for an opponent line from your game not already covered in
  your repertoire.


user3194: Yeah I’ve wanted something like this for quite some time… SO I made one as a side project located here:
http://www.chessply.com
It’s basically an online repertoire manager that lets you organize and train against your rep.
No downloads, just use a modern web browser or mobile device and you should be good to go.  It's very much beta'ish so let me know if anything weird happens :)

user1275: I am not aware of any Online Chess training website which allows you to load your own Chess opening repertoire in pgn format.
Therefore, I added this option in my iChess Android app.
You can load your Opening PGN file with short repertoire lines, and make moves for "your" side and the App will award points accordingly.
More details on this blog: http://mychessapps.blogspot.in/2013/02/how-to-practice-chess-openings-with.html
Regards,
Asim
Developer of iChess App

user9636: I think that openings.chessbase.com has what you're looking for.
I use it to drill openings and it works really good.
Enter moves on the board or click them in the Live Book.
Mark a move to add it to your White or Black repertoire. All marked moves will be colored in the Live Book.
Load your White or your Black repertoire as game notation. White and Black are stored separately.
From any position, drill your repertoire. It doesn't matter whether you have marked moves or not, you can drill any position. The program will lead you into your openings.
If you run out of moves, stop the drill and check the Live Book.
Repeat the drill from the position where you last started it.

user9621: www.chessable.com just launched. This should be exactly what you are looking for. You can import pgns and can train the opening using space repetition. 

user3594: 2018 update:

It seems lichess.org offers a lot of the features you're looking for. On lichess, go to tools then choose Analysis board:

The editor allows you to import FEN, PGN or edit the board yourself. 
You have access to two proper opening databases, one extracted from the games played on lichess and one from the masters games (top players). It's not only an opening explorer showing the lines, but it also shows you samples of games where a given line is employed.
You can set up your position and play through the opening branches, while simultaneously verifying the engine assessment on the position. From a given position you can also choose to practice against the computer, for instance for a variation you've come up on your own.
Last but not least, once your analysis is done, you can save it all as a study (which can also be made private). As such, all the variations and side-lines you've explored/tried and plan to keep will be saved together with the engine analysis of them. These will be your personal repertoires, which you can continuously modify whenever you find improvements.

The features are available to everyone. Once you've created a couple of repertoires and started practicing them on lichess, in your profile there is an analytics page where all sorts of statistics will be calculated based on your performance. For example, your rate of win with a certain line, or your frequency of certain moves and so on. That will help you to assess how you are performing with your different openings.

Older suggestions:
In terms of simple online opening explorers, there's always the Shredder Opening Database, can be useful for quickly checking something.
Then there's the database available on chesstempo which is actually really detailed (statistics on each opening etc) and gives exemplary games of any opening played by real players. 
But I believe for what you want to do, lichess is the go-to choice here. 

618: <p>There are a lot of websites to train your tactics, but I can't find any for opening repertoire. I am looking for something similar to 'Chess Position Trainer' but online, where I can import a pgn file with my repertoire, indicate that I want to train it as Black and track my progress. Does such a software exist?</p>

<p>UPDATE/NOTE: being able to train against my own reperoire, and rather than the best tree/stats move is vital for me.</p>
 8309: <p>Chess.com has a puzzle mode in its chess diagram editor.  Perhaps you could create puzzles for each line in your repertoire, and save them to a group forum.</p>
 34193: <p>I created a web application to help build and memorize chess repertoires.</p>
<p>It is completely free and you can use it from any device.</p>
<p>The app is still in development so everything is far from perfect, but I still wanted to share it because it can be useful to you as the key features (build &amp; train) are functional.</p>
<p>I am interested in having feedback to continue in the right direction.</p>
<p>I am not a company and I am alone in managing this project, I apologize in advance for any problem you may encounter while using it, I will try to do my best.</p>
<p>Feel free to try here <a href="https://chess-repertoire-companion.com" rel="nofollow noreferrer">https://chess-repertoire-companion.com</a></p>
<p>Any kind of help around this project would be greatly appreciated! </p>
<p>Cheers</p>
 3456: <p>Actually, I am the author of a website tool that answers the question to some extent. You can load your own PGN file and train yourself against it as a one-time exercise with no progress tracking. You can replay the exercise by revisiting the dynamic link that encodes all the chess data.
This <a href="http://www.apronus.com/chess/puzzle/?p=0&amp;N=0&amp;w=tjcapqstivohi-tjcapqsticozsgp-tjcapqssinp-tjcapqssizoxe-tjcapqssixa-tjcapqspHdiva-tjcapqspHdicove-tjcapqspHyibocshp-tjcapqspHyinajuftkiqreu-tjcapqspHyinajuftdaeO-tjcapqspHyinampyp&amp;h=%3Cb%3ERefute%20the%20original%20Hanham%20Variation%20of%20the%20Philidor%20Defense%3C%2Fb%3E%0A&amp;m=9" rel="nofollow noreferrer">sample link</a> shows a demo. The task is to play White against the Hanham concept in the Philidor defense, which is considered refuted by opening theory.</p>

<p>I wrote the tool for myself and never found the time to provide a user guide but it should work for everyone once you learn how to operate it. I can help with any questions.</p>
 15847: <p><a href="https://sourceforge.net/projects/chessx" rel="nofollow noreferrer">ChessX</a> might be something you are looking for, it has a training mode that uses a PGN file that you give to it.</p>
 3456: <p>There is a new offering from ChessTempo.com!</p>

<p><a href="https://beta.chesstempo.com/opening-training" rel="nofollow noreferrer">https://beta.chesstempo.com/opening-training</a></p>

<blockquote>
  <p>Welcome to the Chesstempo opening training tool. The opening trainer
  allows you create and manage your opening repertoire, and then train
  against your repertoire using spaced repetition to optimise your
  learning. The opening training also integrates with our play chess
  online feature, allowing you to get feedback after each game on where
  you deviated from your opening repertoire, or extend your repertoire
  to account for an opponent line from your game not already covered in
  your repertoire.</p>
</blockquote>
 3194: <p>Yeah I’ve wanted something like this for quite some time… SO I made one as a side project located here:</p>

<p><a href="http://www.chessply.com" rel="noreferrer">http://www.chessply.com</a></p>

<p>It’s basically an online repertoire manager that lets you organize and train against your rep.
No downloads, just use a modern web browser or mobile device and you should be good to go.  It's very much beta'ish so let me know if anything weird happens :)</p>
 1275: <p>I am not aware of any Online Chess training website which allows you to load your own Chess opening repertoire in pgn format.
Therefore, I added this option in my iChess Android app.
You can load your Opening PGN file with short repertoire lines, and make moves for "your" side and the App will award points accordingly.</p>

<p>More details on this blog: <a href="http://mychessapps.blogspot.in/2013/02/how-to-practice-chess-openings-with.html" rel="noreferrer">http://mychessapps.blogspot.in/2013/02/how-to-practice-chess-openings-with.html</a></p>

<p>Regards,
Asim
Developer of iChess App</p>
 9636: <p>I think that <a href="http://openings.chessbase.com" rel="noreferrer">openings.chessbase.com</a> has what you're looking for.
I use it to drill openings and it works really good.</p>

<p>Enter moves on the board or click them in the Live Book.</p>

<p>Mark a move to add it to your White or Black repertoire. All marked moves will be colored in the Live Book.</p>

<p>Load your White or your Black repertoire as game notation. White and Black are stored separately.</p>

<p>From any position, drill your repertoire. It doesn't matter whether you have marked moves or not, you can drill any position. The program will lead you into your openings.
If you run out of moves, stop the drill and check the Live Book.
Repeat the drill from the position where you last started it.</p>
 9621: <p><a href="http://www.chessable.com" rel="noreferrer">www.chessable.com</a> just launched. This should be exactly what you are looking for. You can import pgns and can train the opening using space repetition. </p>
 3594: <h2><strong>2018 update:</strong></h2>

<hr>

<p>It seems <a href="https://lichess.org/" rel="noreferrer">lichess.org</a> offers a lot of the features you're looking for. On lichess, go to <code>tools</code> then choose <code>Analysis board</code>:</p>

<ul>
<li>The editor allows you to import FEN, PGN or edit the board yourself. </li>
<li>You have access to two proper opening databases, one extracted from the games played on lichess and one from the masters games (top players). It's not only an opening explorer showing the lines, but it also shows you samples of games where a given line is employed.</li>
<li>You can set up your position and play through the opening branches, while simultaneously verifying the engine assessment on the position. From a given position you can also choose to practice against the computer, for instance for a variation you've come up on your own.</li>
<li>Last but not least, once your analysis is done, you can save it all as a <strong><a href="https://lichess.org/blog/V0KrLSkAAMo3hsi4/study-chess-the-lichess-way" rel="noreferrer">study</a></strong> (which can also be made private). As such, all the variations and side-lines you've explored/tried and plan to keep will be saved together with the engine analysis of them. These will be your personal repertoires, which you can continuously modify whenever you find improvements.</li>
</ul>

<p>The features are available to everyone. Once you've created a couple of repertoires and started practicing them on lichess, in your profile there is an <a href="https://lichess.org/blog/VmZbaigAABACtXQC/chess-insights" rel="noreferrer">analytics</a> page where all sorts of statistics will be calculated based on your performance. For example, your rate of win with a certain line, or your frequency of certain moves and so on. That will help you to assess how you are performing with your different openings.</p>

<hr>

<p><strong>Older suggestions:</strong>
In terms of simple online opening explorers, there's always the <a href="http://www.shredderchess.com/online-chess/online-databases/opening-database.html" rel="noreferrer">Shredder Opening Database</a>, can be useful for quickly checking something.</p>

<p>Then there's the database available on <a href="http://chesstempo.com/chess-openings.html" rel="noreferrer">chesstempo</a> which is actually really detailed (statistics on each opening etc) and gives exemplary games of any opening played by real players. </p>

<p>But I believe for what you want to do, lichess is the go-to choice here. </p>
