The Roblox multiplayer game can do this easy.
You need to download the template on the development page.
You can modify it as you wish and then put it through your account to other users or sell it with roblox money.
The most interesting part is that you can show up with your new creations and the other players in the game.
Another tutorial can be found at wiki roblox webpage.
It your work don't show good try the shirt upload page.
You have to wait a few minutes for your avatar to show your creation correctly.
Some of my work can be found with my user : catafest .
You can get a t-shirt with Fedora linux user design from here, take a look:
