Since eBay has now joined the developer contest fray it appears that the Coolest App contests are fast becoming a marketing staple in the battle for the hearts & minds of developers. There are at least four under way at the moment from eBay, Microsoft, and Amazon, as well as a couple that just concluded. Two contests have top prizes of $5,000. In order to help you keep track of this I've added a Contest Tracker to the site. You can get there via the 'Contests' link on the left navigation of this blog or by going to http://programmableweb.com/contests.
It's divided into two sections: the first has in-progress contests and the second lists completed or to which entry is now closed. The latter can help give you some idea of what has worked for people in the past. Good luck.