I look at a bunch of comics (newspaper published and web-only) here's my favorite web-only comics:
Angst Technology - on hiatus, not sure when he's coming back... http://www.inktank.com/
The Chopping Block - Definitely for the sick & twisted... http://www.choppingblock.org/
Dork Tower - Although published in "Dragon" magazine, I found this one on the web first. It motivated me to try my first attempt at web comics (See the bottom of my list). http://www.gamespy.com/comics/dorktower/
GU Comics - Found this one when I first started playing EQ about 4 years ago... http://www.gucomics.com/
Nodwick - Like Dork Tower, also published in "Dragon" magazine and I found it first on the web.
The Weird Worlds of Pewfell Porfingles - I had the very good fortune of having Chuck Whelon the creator of this strip stop by my own website and give me some kind words of encouragment. About a year later I got to meet Chuck at an Alternative Press Expo (APE) in San Francisco. Turns out we live fairly close to each other in the city. However, I haven't had the time to stalk him...yet!
Not only is he a terrific artist, he's also a really nice guy. http://www.pewfell.com/index.html
PVPonline - The best of the best IMHO and consistently funny on a daily basis. I'm also greatly impressed that he was one of the first web comic creators to actually make a living at it. http://www.pvponline.com/
Sheldon - Originally a web-comic but not too long ago actually got picked up by a comic syndicate. http://www.comics.com/comics/sheldon/index.html
Sinfest - Another great one that I've been following for a long time. http://www.sinfest.net/
Voices in My Hand - Another one of my faves. Again, definitely for those that enjoy the sick & twisted... http://www.voicesinmyhand.com/
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Wallop - by yours truly, currently on indefinite hiatus. This was my first attempt (I know, it shows!) and I really enjoyed doing it. Someday I may go back to Wallop, but I want to try something more mainstream...
I've got a new idea that I'm working on that's more mainstream and I hope to put it up on the web once I finish my current contract job. Hey, gotta have something to do in-between job hunting, pc game playing, exercising and sleeping!
http://wallop.keenspace.com/