A flash-based web site for creating your own comic book from a collection of pre-rendered panel layouts, backgrounds, characters, and objects. Actually pretty impressively flexible.
43 things, an user community based around things people are doing and/or want to do, has this page on creating comics. Lots of accounts of people's experiences creating their own comics.
This tool is really cool. You can create multi-panel web comics with this intuitive little tool. Choose from a palette of characters and objects. Very funny, easy and flexible.
This is a nice and reasonably priced app (site includes a downloadable 30-day trial too). Currently Mac-only, but they are apparently working on a WinXP port.