Recently I posted on some recommendations that were sent in on the Change.gov Citizen's Briefing Book. Here's more from that site. Apparently a lot of people are interested in the prize approach, in particular for energy applications. (I'd say that space and a few other areas are even more fertile ground for prizes than energy for a couple reasons, but there's certainly plenty of room for quite a few more energy prizes, too). I guess I'd like to "vote up" some of these, but it looks like it's too late.
Govt Sponsored X-prize for Private Space Industry
An X-Prize for a 50kWhour UltraCapacitor
X-Prize for Green Cars: stop subsidizing over bloated and out dated auto makers!
Ongoing Technology / Innovation X Prize
Energy Innovation Prize
Rewards for Performance, Not Subsidies
An X-prize for next gen environment friendly cars
$1 Billion Solar Airplane X-PRIZE
Inspire Science and Innovation Activity