Spreadshirt members can set up shops, customize, and sell their own t-shirts, as well as a rather large selection of associated items and accessories. Free membership, no up-front cost, easy to set up.

Cafepress offers a slightly more-developed version of the system that Spreadshirt provides, but with the exception that Cafepress is the more recognized authority on custom tees. Same as before, setting up your shop is very easy, and you can customize licenseplate frames, mugs, mousepads, and yes, t-shirts (etc etc etc). Free to sign up. Upgrades available.

Threadless will award you with money for your designs if they’re chosen to be printed. There’s competitions and such. You should really check this one out if you love to design.

TypeTees is a part of the Threadless network, but the concept is so awesome! There are literally hundreds of awesome expressive T-shirts here. Join the Threadless network and you can submit your masterful creations and get some cash.

Spoonflower will let you design your own fabric, and will print it for you! That’s pretty much it, in a nutshell. Check it out, there’s hundreds of custom fabrics already made you can choose from, or create your own. Very awesome.