I just finished working on a module to automatically post news stories from a client web site to Twitter and Facebook. I know that’s not a big deal, but it was kind of cool, and it took a bit of effort to get the Facebook thing figured out. (See this stackoverflow.com post for some detail on that.)
I used the Facebook C# SDK for the Facebook stuff, Twitterizer for the Twitter stuff, and EasyHttp to access the bit.ly API, for URL shortening. And of course I pulled them all into the project with NuGet. I feel like the master of all social media APIs now.