minutes of a programmer
The not-for-profit Social Actions just launched the Change the Web Challenge. The idea is to come up with the greatest application you can think of that leverages their API.
Their API allows you to search a database of social actions -- things like volunteering opportunities, fundraising events, petitions -- aggregated from contributors like kiva.org or idealist.org.
For a quick peek at what you have access to, you can try my very own search for opportunities. The code for it is available as a git repository.