I have a cafepress site and love it but you have to be able to create a design and save as a png or jpeg file. Photoshop or Corel draw.
Zazzle is good because you can just type something on the item with their program, thing is though someone can steal it by typing it up themselves and saving money, you never see a cent.
Being an affiliate for either will give you a return. Cafe press is 15% through Commission Junction and Zazzle is 7% direct. There are lots of different stores like this, e shirt threadless... there is a squidoo lens with a few more listed.. under cafepress competition.
Speaking of squidoo, you can set up a lens and earn affiliate income by showcasing cafepress, amazon, ebags, and other stores things on your lens. You can always promote my cafe press stuff...

honest I won't care promote away...

just kidding...
For an example though of how a lens works and so you can see how products are promoted you can go to:
Support and Resources for WAHMs and those considering it.I have a handful of different lenses on various topics, heck my 8 year old has a couple, it is really easy. You also earn adsense royalties and affiliate royalties, although I don't expect it is very much, I think you would need a lot of lenses to make any real return. But sometimes every little bit helps.