Double Fine's first "Amnesia Fortnight" event resulted in prototypes that became all the company's recent games: Costume Quest, Sesame Street: Once Upon a Monster, Stacking, and Iron Brigade. For the latest iteration of the internal game jam, the company has opened up the process to you!
No, you don't get to make prototypes for Double Fine games. Instead, you can put your money towards a vote on 23 different ideas, in order to pick the four that will be made into prototypes by the studio. The voting process is hosted by the Humble Bundle, and the money goes to Double Fine, Child's Play, or the Humble Organizers, in a proportion of your choice.
Contributors will get the finished prototypes when they, you know, exist, and they'll get downloadable prototypes of Costume Quest and Happy Song (which eventually became Sesame Street: Once Upon a Monster) now.