I like chess24.com....there are a lot of videos you can watch on various openings. Lots of games if you like fast (blitz) type games...good live stream coverage of major tournament, etc.
Tim Brennan's "Tactics Time" blog / newsletter:
http://tacticstime.com/
For openings - very useful if you end up with a lousy position after just a few moves and want to see where you went wrong:
http://www.365chess.com/opening.php
For playing over old games (new games are there as well, but I mostly use it for older ones):
http://www.chessgames.com/
For amazing stories about the history of chess and chess players:
http://www.chesshistory.com/winter/
Streetfighting Chess. Proving that the art of annotating chess games is not dead!
http://www.streetfightingchess.com/
Another chess tactics server. I haven't used this for a while as I tend to play blitz games instead:
http://chess.emrald.net/