You've got to respect the American Red Cross: they're just a bunch of people who love to help people. As a recipient of their assistance through multiple emergencies (hey forest fires, you suck) I've been delighted to see their various disaster apps pop up on the Play Store. But their latest tops them all: Team Read Cross gives you a ton of information on volunteer opportunities in your area, then "gamifies" the act of completing them, sort of like a Foursquare that actually encourages you to do meaningful stuff.
If you've spent any time gaming on Android, you probably remember OpenFeint. Nearly every major game integrated it in some way, usually allowing players to log in with a single username, collect achievements, and post scores to a global leaderboard. It was handy for what it did, but if you didn't care about competing, it felt a lot like obnoxious spamware. Unsurprisingly, it closed down in December of last year. Today, however, it's being sort of reborn as OpenKit, a project headed by one of the co-founders of the original service.