user618: The only 7 piece site I heard about is http://tb7.chessok.com/ , and upon registration I am able to setup various positions with 2-6 pieces . However as soon as I setup a position with 7 piece, I get an error about service not being avaiable. Has anyone had better luck or know of any other site with 7 pieces tablebases?
Update: in the end I discovered that FinalGen solved a vast majority of positions I was interested in, included those with more than 7 pieces. http://finalgenchess.ovh/home_ing.php

user16979: https://syzygy-tables.info/ allows to look up 7-men endgame positions. It does not give DTM information for 6- and 7-men endgames, only DTZ50+. On one hand, it is compliant with 50-move rule, OTOH, DTZ50-optimal play can be ugly (if it is possible to push a pawn or sacrifice a piece, it will).
Also available for download: https://tablebase.lichess.ovh/tables/standard/

user2142: The 7 piece EGTB is not yet freely available online, as far as I know. Building the tablebase is costly in time and resources and the storage space is not cost-free either.

Concerning http://tb7.chessok.com/ do not be too disappointed, because they mention this explicitly (at least today, maybe not on April 11th when you tried):
 "Currently the tablebases are only accessible by legal owners of Chess Asssitant 14, ChessOk Aquarium 2014 or Houdini 4 Aquarium. Later, more users will be able to use the service."

And on their signup page: 
" After registration you will be able to:
probe 6-man ending tablebases; 
(...)
probe 7-man tablebases (license key required – see the next page)."

user3951: http://chessok.com/?page_id=27966
History Note
The first ending tablebases - for all 4-piece endgames - were built by the end of the 80-s. In the beginning of the 90-s, the same task was done for 5-pieces. In 2005, 6-piece endings were solved in Nalimov Tablebases which are now used by many professional chess programs and services.
Experts didn’t expect 7-piece endings to be cracked and catalogued until after 2015, but Convekta Ltd, namely programmers Zakharov and Makhnichev - the developers of the Aquarium interface - managed to solve this task in just 6 months using a new algorithm designed specifically for this purpose and run on the Lomonosov supercomputer based in the Moscow State University.
Lomonosov Tablebases
As a result, we now have 525 tablebases of the 4 vs. 3 type and 350 tablebases of the 5 vs. 2 type. The calculations for 6 pieces playing against a lone king weren’t done because the outcome is rather obvious.
The total volume of all tablebases is 140 000 gigabytes, which is obviously too much for personal computers. Lomonosov Tablebases will be accessible online from the Aquarium interface and on the chessok.com website. All users of ChessOK Aquarium, Houdini Aquarium and Chess Assistant products get free access to the service for the period specified in their product description.

user9030: Original Answer:
7-men Lomonosov is free as an Android application.

Update - September 2018:
7-men Syzygy is available for free. Also available from Lichess.

618: <p>The only 7 piece site I heard about is <a href="http://tb7.chessok.com/" rel="noreferrer">http://tb7.chessok.com/</a> , and upon registration I am able to setup various positions with 2-6 pieces . However as soon as I setup a position with 7 piece, I get an error about service not being avaiable. Has anyone had better luck or know of any other site with 7 pieces tablebases?</p>

<p>Update: in the end I discovered that FinalGen solved a vast majority of positions I was interested in, included those with more than 7 pieces. <a href="http://finalgenchess.ovh/home_ing.php" rel="noreferrer">http://finalgenchess.ovh/home_ing.php</a></p>
 16979: <p><a href="https://syzygy-tables.info/" rel="nofollow noreferrer">https://syzygy-tables.info/</a> allows to look up 7-men endgame positions. It does not give DTM information for 6- and 7-men endgames, only DTZ50+. On one hand, it is compliant with 50-move rule, OTOH, DTZ50-optimal play can be ugly (if it is possible to push a pawn or sacrifice a piece, it will).</p>

<p>Also available for download: <a href="https://tablebase.lichess.ovh/tables/standard/" rel="nofollow noreferrer">https://tablebase.lichess.ovh/tables/standard/</a></p>
 2142: <p>The 7 piece EGTB is not yet freely available online, as far as I know. Building the tablebase is costly in time and resources and the storage space is not cost-free either.
<br>
<br>Concerning <a href="http://tb7.chessok.com/">http://tb7.chessok.com/</a> do not be too disappointed, because they mention this explicitly (at least today, maybe not on April 11th when you tried):
<br> "<em>Currently the tablebases are only accessible by legal owners of Chess Asssitant 14, ChessOk Aquarium 2014 or Houdini 4 Aquarium. Later, more users will be able to use the service.</em>"
<br>
<br>And on <a href="http://tb7.chessok.com/signup">their signup page</a>: 
<br>" <em>After registration you will be able to:
<br>probe 6-man ending tablebases; 
<br>(...)
<br>probe 7-man tablebases (license key required – see the next page).</em>"</p>
 3951: <p><a href="http://chessok.com/?page_id=27966">http://chessok.com/?page_id=27966</a></p>

<p>History Note</p>

<p>The first ending tablebases - for all 4-piece endgames - were built by the end of the 80-s. In the beginning of the 90-s, the same task was done for 5-pieces. In 2005, 6-piece endings were solved in Nalimov Tablebases which are now used by many professional chess programs and services.</p>

<p>Experts didn’t expect 7-piece endings to be cracked and catalogued until after 2015, but Convekta Ltd, namely programmers Zakharov and Makhnichev - the developers of the Aquarium interface - managed to solve this task in just 6 months using a new algorithm designed specifically for this purpose and run on the Lomonosov supercomputer based in the Moscow State University.
Lomonosov Tablebases</p>

<p>As a result, we now have 525 tablebases of the 4 vs. 3 type and 350 tablebases of the 5 vs. 2 type. The calculations for 6 pieces playing against a lone king weren’t done because the outcome is rather obvious.</p>

<p>The total volume of all tablebases is 140 000 gigabytes, which is obviously too much for personal computers. Lomonosov Tablebases will be accessible online from the Aquarium interface and on the chessok.com website. All users of ChessOK Aquarium, Houdini Aquarium and Chess Assistant products get free access to the service for the period specified in their product description.</p>
 9030: <h3>Original Answer:</h3>

<p>7-men Lomonosov is <a href="https://play.google.com/store/apps/details?id=com.convekta.android.lomonosovtb" rel="noreferrer">free as an Android application</a>.</p>

<hr>

<h3>Update - September 2018:</h3>

<p>7-men Syzygy is <a href="https://syzygy-tables.info/" rel="noreferrer">available for free</a>. Also <a href="https://lichess.org/blog/W3WeMyQAACQAdfAL/7-piece-syzygy-tablebases-are-complete" rel="noreferrer">available from Lichess</a>.</p>
