CGTrader is a site that I considered when planning to sell my 3D models for 3D printing. It's still an option if Ponoko ever falls out of favor, but CGTrader is less about 3D printing and more about, just, 3D which I think would be confusing if my Mom were shopping for models for her theoretical $300 3D printer. But CGTrader is good and I have nothing bad to say about them.
CGTrader is having a Valentine themed design contest. If you can already do 3D design you can enter and win... wow, a lot of great prizes including a 3D printer. And if you can't do 3D design I'll have contest for you soon.