Elastic powered airplane and Elastic crossbow by Sublime.
Nathan Patterson, aka Thingiverse user pattywac, has organized a pair of user-funded design contests and is soliciting entries and prize donations for each. The first, with a current prize pool of $100US, is to design the cleverest MakerBot-able object that incorporates a rubber band. Two of my favorite entries so far are shown above.
The second contest, to design the most useful improvement for the MakerBot itself, is also ongoing, and currently has a prize pool of $65US. Both contests end on Sunday, November 14. [Thanks, Nathan!]