If you're looking to give back to your community or volunteer for a charity, a new site from Google can help you find the right opportunity. It's called AllforGood.org and it's a volunteer search site.
When you visit the site, it detects your location via your IP address. The results will return opportunities from a variety of organizations.
The site was built on 20% time with App Engine and Google Base. An API is also provided for third party developers.