Ah.... it's been forever since I've read any webcomics, but I still have my list. I'll catch up on all of them someday... really...
You're already reading a couple of my old favorites, and a couple others have been mentioned, but here are some others I liked.
Acts of Gord -
http://www.actsofgord.com/index.html
(Not really a webcomic, but still funny enough for a good read. Doesn't update anymore.)
Bob and George -
http://www.bobandgeorge.com/
(Very much like 8-bit Theatre, but with Megaman)
Bruno the Bandit -
http://www.brunothebandit.com/
Dragon Mango -
http://www.dragon-mango.com/main.htm
Gaming Guardians -
http://www.gamingguardians.com/
(Also has several sub-comics)
Girl Genius -
http://www.girlgeniusonline.com/
Mab's Land -
http://www.mabsland.com/index2.html
Mac Hall Comics -
http://www.machall.com/
(Also the creator of my favorite Morrowind mod ever - Mog Mod)
Order of the Stick -
http://www.giantitp.com/
Snafu Comics -
http://www.snafu-comics.com/
(Actually a collection of 10 different webcomics, some are finished, some are still running)
The Fortuna Saga -
http://www.fortunasaga.com/index.php
Zap! -
http://zapinspace.com/
Zebra Girl -
http://zebragirl.keenspot.com/
(Doesn't actually have anything to do with zebras
)
...
Yeah, I used to read WAY too many webcomics...