This is a public service announcement for anyone left out there who has not been to Threadless.com to pick up their mouse, and go click your bootie over there.
I am not sure what to compare Threadless to - design karaoke? How it works is you can submit you own tee shirt designs, and vote on others. If you "win" they will run your design on the shop.
It's really fun, and in an age of never-endingly clever tee shirt graphics on all your hipster friends, hopefully this will be a good resource for you to get with the program ;-)