Oh goodness. If we're including webcomics, then I have to include Penny Arcade's.
Oooh, and Questionable Content!
I've seen some other webcomics too, but they usually can't hold my attention enough for me to bother checking back. QC's funny to me, but there's a bit too much indie-music-humor and sex-related-humor for it to really be any more successful than it is.