Um let's see here... in alphabetical order:
Evil Diva, a cutsey story about an eleven-year-old devil named Diva with a problem... A
goodness problem. The art is cute and colorful, and the story seems fun thus far.
Girl, a story of a girl and her sidekick who do... things. Strange things. Giant personifications of the concept of rhythym being defeated by a girl playing Guitar Hero and her half-sister rapping-type-strange. Wacky and somewhat messed up. The art's really cool. Contains lesbians <3
Head Trip, currently inactive but funny. It has people. They do things. What more do you need? (read:I can't remember much about it as it's been a while since it was updated)
I Was Kidnapped By Lesbian Pirates From Outer Space, a salacious tale of seduction and lust. Pretty much exactly what it says on the tin- Susan Bell, an office worker, is kidnapped. By pirates. From outer space. And they're all lesbians. The title is a mouthful, but the story's funny and the characters are endearing.
Misfile, the story of a girl who was a boy, a girl missing two years of her life, and the pot headed angel that did it to them. The plot is rather intricate on this one, which I like. The art's good, though it's kinda stiff at the beginning.
Questionable Content, which is basically two parts drama, one part NSFW, and over nine thousand parts hilarious. The characters are awesome~ But it's kinda hard to figure out what to say about it.
Red String, a romance and love themed comic where drama is the name of the game. I love the plot and characters, even if a recent twist makes me want to painfully gouge the eyes out of a particular one. Notable for, despite being written and drawn by a very American artist, being set in Japan fairly convincingly. Or at least, since I'm not really familiar with Japanese culture outside of Anime, Manga, and a bit of research, it's had a lot of research put into it. Also has lesbians. For some reason I read a lot of web comics with lesbians.
Sparkling Generation Valkyrie Yuuki, which other people have already pretty much described well enough. Yeah, I'm lazy.
Sore Thumbs an indescribably strange web comic that deals with both politics and video games. See that new page? That's the result of a nuclear bomb being shoved up a large, demonic, female Osama Bin Ladan's asshole to destroy her.
Springiette, a webcomic about a ninja, her potato chips, and the pirate that keeps her feet on the ground (usually). It's funny and wacky and weird.
Yu+Me: dream, a very surreal lesbian webcomic. Starts out as a regular high-school drama only with lesbians, and it gets... stranger over time. Probably my favorite webcomic ever, although your chances of still liking it after the end of part one are only about 50/50.