Probably two years ago I got an idea to create a site that would allow people to upload images, get that image printed on shirts, and sell that design to other people. This was primarily in response to a need that I had at the time, and it sounded so good in that it matched with the ideas of the Cluetrain Manifesto. At the time I only found a single competitor and decided not to pursue the idea. Startup and labor costs also factored into my decision, but I'm shocked at how popular the idea was. In the order that I found them, here are companies that offer on demand printing of apparel and other random things:
I have personally bought a shirt from Zazzle because I wanted to check out their quality. It's faded a bit in the time that I've had it, but still holding up. SpreadShirt appears to be the only company that can print in white which puts them on top of my personal list, and I have yet to completely check out CafePress.
All in all, I haven't regretted my decision to leave this as an idea, but I'm sure glad someone else decided differently.
