You too can have a webcomic
xkcd - A webcomic of romance, sarcasm, math, and language. Simple art.
dinosaur comics - Philosophy, science and sexiness. Very simple art.
Giant In The Playground
Two comics about fantasy gaming:
Order of the stick
Erfworld:The Battle for Gobwin Knob
Inverloch - Mostly standard setting with Wizards, Elves, and, these other things...
Earthsong - Innovative setting, all manner of races, most notably planets and stars.
Gone with the Blastwave - Postapocalytic humor in a pointless war.
Buttercup Festival - This is dark in a bizarre, funny way. Sadly it hasn't been updated in a while.
Sinfest - God, Satan, Drugs, Sex, Books
This is an amazing and beautiful webcomic done in watercolors.
I was introduced to it with Minus 37, which changes meaning somewhat once you understand what the story is really about.