The Adform Android SDK by rprunskas allows to implement advertising features into mobile applications. Basic antegrations include inline ad, overlay ad, intersitial ad, adhesion ad, and video ad. Resources include advanced integrations and release notes.
Have you noticed the many tools to share your location? There's FourSquare and Gowalla, both about a year old. Then there are old-timers like Brightkite. And now, there's one that works with all three (and a couple more).