I am rather a fan of webcomics (anything that entertains me and is free is good) so here's a bunch of links to the ones I read regularly. The order they're in is just the order they happen to be open in my browser -- no real significance is attached.

Schlock Mercenary
Military SF humour. Invariably entertaining, and not afraid to kill off major characters. Currently my favourite place on the web.
User Friendly
Techie humour about people working at an ISP. Been around a long time, and consistently funny.
General Protection Fault
Techie humour (mostly) but long story arcs. Gets a little tedious at times, but always bounces back.
Piled Higher and Deeper
Grad student humour. Updated irregularly, but worth the wait.
Penny Arcade
Gaming comic, a bit vulgar at times. Still good fun, though, although perhaps not one to show the kids.
Kevin and Kell
Furry, been around a long time. In general, I'm not a fan of furries, but K&K often has a very interesting take on things.
Hard to classify -- more an attempt at social commentary than anything else. Despite the name, the strip is fairly tame (much more so than Penny Arcade, for example) but does take digs at politics and religion a lot. Also occasional bad language, so probably no kids for this one either.
Real Life
Another one that's hard to classify. Having some server issues at present, so keep trying.
"This is Jeremy, just turned nine, youngest son of Frankenstein" -- the tagline says it all. Usually fun, very irregular.
Techie/academic humour. About Gav, a Nuclear Engineering student. Lots of giant robots and things.
A rather odd one, but lots of fun. Expect to see things catch fire frequently.
Chopping Block
Dark single panel about a serial killer named Butch. Another one you probably don't want to show the kids, but incredibly funny. Has recently had a few long gaps between updates, but seems to be up again.
This one is about a fictional gaming magazine. PvP stands for "Player versus Player", in case you didn't already know.
Daily Yahoo comics
A collection of comics pulled from Yahoo's comics pages, conveniently collected in one place. Script written by Rooijan with some minor modifications by me. Update: Fixed. They've changed their page layout and URLs. New URLs and REs in the script and everything works fine again.
21st Century Fox
A scifi furry, mostly romantic themes. Not usually my kind of thing, but the story's well told and it's fun.
Another (sort of) gaming comic with lots of black humour. Occasional violence and profanity, but not as much as Penny Arcade. One of my current favourites.