Some of my favourites:
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/
Blog 4
I kniow the guy that does this. He is very handsome.
Probably the handsomest of all the handsome chess players.