A challenger appears!
I haven't sold any models yet, but there has been a few alternatives to Ponoko that I may throw my hand in.
Maker6 allows you to sell models, physical prints, and eventually print time on your machine. So basically it's a little bit of Ponoko, Etsy, and MakeXYZ rolled into one, more focused on 3D printing. If it offered me the chance to sell my talents as a designer it'd be all I ever wanted. And it's got a more 3D printerly name that's easier to remember. The cons is that it takes 9% of all sales, including designs, where as Ponoko takes nothing for designs, and it isn't set up for free giveaway items. On the other hand it's still new and in active development so maybe they'll take my feedback and make that possible.
My Mini Factory has been trying so hard to promote themselves that they've bordered on spam. They're paying designers to come up with a number of unique models that I haven't seen anywhere else which has prompted me to check them out, only to discover that when you say you want to download something they respond by e-mailing you the download link. A bit hinky in my opinion, but not too uncommon I guess. They're running a competition with a grand prize of a free Up! mini for a chess set. I've had good luck with chess sets in the past and I've got a couple that I'm working on now so I may throw in my hand. I'd love to check out another 3D printer. Then, who knows, I may run a contest of my own.
By the way, contest there. Design a chess set, win a 3D printer. Enter now!