A lot of mine have already been posted. YTMND, Newgrounds, Homestar Runner, 8-Bit Theater, 4chan, and Mondo Mini-Shows.
Also Cyanide and happiness used to give me a good chuckle, and xkcd is one I still visit from time to time. Penny Arcade, FantasyAnime before they had most of their stuff taken down due to copyright, Lemon64, Cracked, The Onion, Bartleby (I think I stopped going because I've read pretty much everything there), Extreme Tech, Scientific American, National Geographic, VGMusic and OC Remix, Least I Could Do, gregcomic, and probably dozens of others that I'm forgetting.