The Gmail API SDK for .NET contains authentication and authorization with Api Keys and OAuth 2. Examples demonstrate a Google Discovery API sample along with web applications, service accounts, and installed applications.
Yahoo is weighing its options with its blog-based social network tool MyBlogLog and has not ruled out closing down the site--and its API (our MyBlogLog API profile). When the MyBlogLog API launched, it looked to have a promising future, though this was before Facebook and Google released similar features.
Mashup games are always fun. Those built upon FourSquare have an extra layer of enjoyment, because they're tied to the real world. In the case of City Warfare, players use various water weaponry to soak other FourSquare users.