The BBC reports a contest put on by the “Big Green Challenge” in which 10 people will win cash prizes toward turning their green idea into reality.
At their site The Big Green Challenge , they lay out the ground rules and prizes:

This prize is about encouraging change and innovation. We’re challenging you to reduce your CO2 emissions in your community – which can be local, regional or more widely dispersed – by 60%. Are you up to the challenge? Our ten finalists, once selected, will have a year to show us that they can turn their BIG ideas into action. The winner will be announced in late 2009.

Note: You must be a UK based non profit group and you can create one for the sake of the contest BUT it just so happens I found a US based one too!
Check out Going Green . For this you must be between 5 and 19 years old and a US resident.