There are a slew of humorous webcomics out there (Sluggy Freelance* probably being the best-known), but only a few I find consistently funny; of course, given the extreme nature of what I find funny, your mileage may vary considerably. My current favorite is R. K. Milholland's Something*Positive (background here), what with its singularly cynical, dark, and jaundiced view of the world and human nature. I mean, Rippy the Razor ("rippy," as in "It's down the block, not across the street!")? That's some funny stuff right there! Anyway, the panel from this strip had me guffawing loudly enough to scare the cat:
*The beginning series wherein Riff attempts to summon Satan online had me loling and spewing Diet Pepsi all over the place. Of course, we all know now that Satan was already online and had an AOL e-mail address.