Well PvP has already been mentioned. I also follow:
Punch and Pie
http://www.punchanpie.net/ by aerie and chris daily - it is updated sporadically. Great story that I think is semi-biographical of the author, Aerie Daily.
Wayward Sons Legends
http://waywardsons.keenspot.com/ Great SciFi comic that blends in the mythology of earth history in a realistic way. Great story and great art. Updates every weekday with bonus material on Sat and Sun.
Requiem
http://requiem.spiderforest.com/ Fantastic Digital art, great sci-fi story that has been online since 2004. It updates every weekday and the author blends in current topics and science news items into the story. This is more like a graphic novel. Lot of characters and character development so it can be hard to follow at times. It took me a week of reading regularly to catch up and I've been following it for about 2 years now.
The Order of the Stick
http://www.giantitp.com/comics/oots0778.html Great fantasy story based on a D&D campaign. Anybody who has ever role played will love the humor here. Updates once or twice a week but sporadic as to when. Very British type of humor.
Ratfist
http://ratfist.com/page-40/ by Doug Tennapel, creator of Earthworm Jim. This is his first web comic and he is doing a great job. Superhero based world more like The Tick than DC Comics Superheros. Updates weekdays.
XKCD -
http://www.xkcd.com/ Very cerebral and computer programmer oriented but also extremely funny one offs. Be sure to mouse over and read the text there, its often funnier than the panels.
OGLAF - WARNING - Sexual based comedy not work safe or for those easily offended -
http://www.oglaf.com/ As stated fantasy/sexual based comedy. Only updates on Sundays but with more panels than those who update daily. While he has a main ongoing storyline, there are also other storylines here and there within it and it jumps around quite a bit. However, each section is pretty much self contained and can be followed.