Today, alongside the launch of Nexus 4 and Nexus 10, Google is officially making the Android 4.2 Jelly Bean SDK available to developers. Included in the release are the lock screen widgets, the Daydream interactive screensaver mode, enhanced support for external displays, and more. Developers can download the Android 4.2 platform from the Android SDK Manager. Google also has an overview of what’s new on its website and an API overview here.
The full release notes from Google (via the Android Developers Blog) are below: