Microsoft today released the Xbox SmartGlass Software Development Kit (SDK) for developers, providing them with the free tools and information they need to create Xbox 360 games that can interact with Activities, the “second screen” experiences via the SmartGlass mobile app on Windows 8/RT, Windows Phone 8, iOS (iPhone and iPad), and Android.

The SmartGlass SDK is not a public release, per se: It is available only to partners that already have agreements with Microsoft to develop Xbox 360 games or entertainment applications. If you are such a developer, you can download the SDK here.

According to Microsoft, the SDK includes the following features:

Xbox SmartGlass Studio, which enables developers to start building Xbox SmartGlass Activities.

Xbox SmartGlass JavaScript Library. The JavaScript Library contains a set of device APIs that are consistent across all platforms.

Sample Code. A sample application that provides developers with an example of how to build an Xbox SmartGlass Activity.

You can find out more about SmartGlass from the Xbox SmartGlass web site. But since so many people seem to confuse what this thing really is—a mobile app and a platform for connecting those apps to Xbox apps and games—you might also read my Xbox SmartGlass Preview for more information.