XKCD - The original and the best
Dilbert - A comic that should be sent to every boss in the world whenever it comes out.
PvP - My link to gamers
Table Titans - My link to board gamers
Questionable Content - The artist has gotten so much better over the years and the comic has great content themed around acceptance of others not like you.
Saturday Morning Breakfast Cereal - Like XKCD but much more irreverent