As for me, I've got a bunch I'm currently watching:
XKCD
Dresden Codak (though it's kinda gone to shit ever since the author started the most recent extended tale; rarely updated, but a must-read)
Darths & Droids; Star Wars scenes set to a D&D theme; another great one which is fairly popular (and large)
Dr. McNinja; More awesome than bacon.
Girl Genius Steampunk adventure; very long, but interesting
SMBC Similar thematically to XKCD, but with more color and intentionally bad art.
Cyanide and Happiness often NSFW; groan-worthy puns and jokes
Vattu another Evan Dahm webcomic; I highly suggest reading Rice Boy and Order of Tales too; as mentioned before, his webcomics feature excellent worldbuilding in a semi-abstract style
Pictures For Sad Children somewhat depressing, extremely funny
Black Hole Or at least I think that's the name... It's a black & white webcomic in space; it has a large archive, but each one goes really fast (in fact, I recommend spending only a second or two on most pages, as it seems to be meant to flow almost like a stop-motion or film storyboard); interesting story, though I'm not entirely sure where it's all going yet.
Twilight Lady Sort of a World of Darkness vibe to it; it's apparently gone through several volumes with several artists (the link is to volume 4, where I started reading).
Homestuck
One I've finished, and which is a completed webcomic:
Digger I
highly recommend this one; as, apparently, many others do too, since it won a Hugo Award. It is really, really good. The story follows a wombat; who in the story's lore are sort of like pragmatic dwarves. It also has an adorable demonspawn.