I am a fan of:
Partially Clips - a webcomic that takes clip art and makes it funny.
Get Your War On - a webcomic and print comic that takes clip art and makes it funny, as well as angry.
Penny Arcade - because most everyone is a fan already.
The Perry Bible Fellowship - ignore the name, because it has nothing to do witht he strip. The humor outshines other webcomics the way the stars in the sky are outshined by the sun.
Qwantz (Daily Dinosaur Comics) - hundreds of hilarious comics that show just how versatile the same six comic panels can be.
The Order of the Stick - I'm required to read this by law.
And finally, though he only occasionally does web comics,
Jay Pinkerton is the funniest human being on Earth. I participate in a (mostly) weekly 'Write This Comic' contest in the forum.
There are others I read, but you don't get to know about them because they're called 'guilty pleasures' for a reason.