I read many great webcomics, and, as one geek to another, I feel I should give everyone a comprehensive list, so that you can all share in my daily giggles:
1.xkcd is an amazing daily strip with references to math, geek culture, the blogosphere, and many other things. I read xkcd faithfully and avidly over anything else, and if you were to pick one comic from this list, this would be the one.
2.Ctrl+Alt+Del is the staple of all gamer comics, and is wonderful for hardcore gamers, but many of the best jokes are not related to gaming at all.
3.Questionable Content is the comic without a stereotype or category, with jokes that can make absolutely anyone giggle, but it has quite a story, and you may want to spend a night trudging through the archives, which will allow you to understand so much more
4.A Softer Word is a daily short strip that puts captions on fine photography, and which has, on occasion, caused me to fall out of my chair laughing.