Above and beyond the Windows Runtime APIs that you primarily use for writing Windows Store apps, there is a host of functionality available in the Windows Live Services SDK, which connects your app to SkyDrive, Hotmail (email, contacts, and calendar), and Skype (messaging and calling). SDKs are available not only for Windows Store apps, but also for iOS, Android, and Windows Phone.

The Live Developer Center, http://dev.live.com, is the first place to start (and be sure to check out the Interactive SDK). The API docs are here.

For some background, there are a few posts on the Building Windows 8 blog that are relevant:


At //Build 2012 we had these sessions on Live Services:

And these at //Build 2011