Some of my favourites:
Tim Brennan's "Tactics Time" blog / newsletter:
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:
For playing over old games (new games are there as well, but I mostly use it for older ones):
For amazing stories about the history of chess and chess players:
Streetfighting Chess. Proving that the art of annotating chess games is not dead!
Another chess tactics server. I haven't used this for a while as I tend to play blitz games instead: