user109: I like Chesstempo and chess.emrald.net[1].
Are there any others?
Edit (25.12.2020): Since at least 2018, chess.emrald.net has often not been responsive.

user8727: Take a look at this.This chessbase site is pretty useful.

user5653: chessity.com is fun, and the free membership offers 20 puzzles/day and access to multiplayer games. 

user4543: chess24.com 5 free problems a day.

user8402: Lichess.org has both tactics and opening training.

user18: http://www.chess.com/tactics/myhome.html
Free: 5 problems/day 

user142: Chesstempo.com is the one I use. It's pretty excellent. I'd like to hear if there's a better one, though.

109: <p>I like <a href="http://chesstempo.com/" rel="nofollow noreferrer">Chesstempo</a> and <a href="http://chess.emrald.net/" rel="nofollow noreferrer">chess.emrald.net</a>[1].</p>
<p>Are there any others?</p>
<p><strong>Edit (25.12.2020)</strong>: Since at least 2018, <a href="https://www.reddit.com/r/chess/comments/82oyig/is_chessemraldnet_gone_forever/" rel="nofollow noreferrer">chess.emrald.net has often not been responsive</a>.</p>
 8727: <p>Take a look at <a href="http://en.chessbase.com/post/chessbase-s-free-tactics-training" rel="nofollow">this</a>.This chessbase site is pretty useful.</p>
 5653: <p><a href="http://chessity.com" rel="nofollow">chessity.com</a> is fun, and the free membership offers 20 puzzles/day and access to multiplayer games. </p>
 4543: <p>chess24.com 5 free problems a day.</p>
 8402: <p><a href="http://lichess.org/training/" rel="nofollow">Lichess.org</a> has both tactics and opening training.</p>
 18: <p><a href="http://www.chess.com/tactics/myhome.html">http://www.chess.com/tactics/myhome.html</a></p>

<p>Free: 5 problems/day </p>
 142: <p>Chesstempo.com is the one I use. It's pretty excellent. I'd like to hear if there's a better one, though.</p>
