eBay is having their Developer Challenge 2006 until January 31st open to individuals and teams. The idea is to build an interesting application using their Web services API. dW also has a three-part series of articles on eBay's API that could help.
For the individual ranks, judging is based 40% on innovative use of their API, 30% on demo-ready look, feel and stability, and 30% designed for eBay users.
For teams it is 30% innovation, 20% look and feel, 20% eBay ready, and 30% on the quality of collaboration between team members.
Prizes (individual): $5000, $1000, and three iPod Nano winners
Prizes (team): XBox for up to 4 members, and free trip to demo your application at the O'Reilly Emerging Tech conference in March. Two other teams can win up to 4 iPod Nanos per team.