Player vs. Player -Possibly the best online comic ever done, IMHO.
Anywhere But Here (note: Start from the beginning, as of late real life has been interfering with the author's work and starting from the beginning is important to the plotline)
General Protection Fault (author's life is also interfering a little, so it's now updated once a week. Timeline is also very important)
Used to read
User Friendly, but I haven't found it funny in quite some time. Also used to read
MegaTokyo, but the plot got too convoluted to keep up with, so I dropped it as well.
While not exclusively online, I read the following comics from online as well:
For Better or For Worse
Dilbert
FoxTrot
While not a comic, if you're looking for daily computer humor, ComputerWorld's
Shark Tank is for those of us that swim the IT waters.