My personal favorites...
http://dominic-deegan.com/ first and for most. Such an amazing plot in a webcomic? Impossible! Well, not any more!
http://pc.gamespy.com/articles/?constraint.grid.section=Flintlocke's+Guide+to+Aze roth
FLINTLOCKE!! ZOMG ZOMG ZOMG! This is a webcomic about World of Warcraft, and it's hilarious. If you play WoW, this is a MUST READ! Even if you don't play WoW... STILL READ!
http://www.drmcninja.com/index.html
A doctor. That is a ninja. His secretary is a gorilla. What's there not to love?
http://www.cad-comic.com/index.php
Everyone knows what this comic is.
http://machall.com/
I'd like this more if they updated it more often. Still pretty funny.
http://samandfuzzy.com/
A webcomic about a neurotic bear and lonely Taxi cab driver. Pretty damn funny.
http://no4thwall.com/
It's on hiatus, but it's still hilarious. VERY RANDOM.