There are a couple of South Park avatar makers out there, I guess. Here's another:
http://www.southparkstudios.com/fans/avatar/
I am also accumulating a list of sites that let you input a photo (or other image) and manipulate it, e.g.:
http://www.imagechef.com/
http://obamiconme.pastemagazine.com/
Sort of photoshop-lite, I guess, in that you start with an image of your own first. And of course many people just snag an image they like from the web somewhere. (In a sense, that's what I did, though I do own the item I'm using an image of.)
Pshrynk, there are modules that can be added to phpBB (our forum software) that cycle through a set of avatars each time a member posts, but adding a feature like that to the site might be somewhat controversial. I do have an outstanding offer to our Glorious Queen to make her a utility to do that, but I haven't had time or spare brain cells to work on it.