Fair warning: participate for the fun, not for the prizes. Mozilla ran a competition like this in 2010. My game "Favimon" won the "Most Original" category. I never did receive most of the prizes they claimed all winners received, including a copy of Adobe Dreamweaver Creative Suite 5, a guest post on the Yahoo! Games blog, a Think Vitamin sponsorship, and John Resig's "Secrets of the JavaScript Ninja" book. This was despite many polite inquiries to the competition organizer. Overall I thought the competition was excellent, but Mozilla's lacklustre follow-up afterwards was disappointing.