Today, with the official release of the Nexus 4, Nexus 10, and Nexus 7 HSPA+, Google has released the Android 4.2 SDK, "a new and improved Jelly Bean."
Along with the SDK release, Google has made available SDK Tools r21, the Android NDK, and of course some helpful API documents. Highlighting some of the benefits of the new SDK (and, by extension, Android 4.2), Google touts "Renderscript computation directly in the GPU" for the Nexus 10, "a first for any mobile computation platform," lock screen widgets, Daydream, incredibly enhanced support for external displays, and optimizations for international users.
A full list of changes (specifically those pertinent to developers) can be found in the API docs, linked at the end of this post.
If you're a developer who's been waiting for the new development kit to drop to create some awesome 4.2 Jelly Bean-friendly apps, now's your chance. It's worth noting that the SDK Tools r21 package hasn't been listed yet on the Android tools download page, but you should be able to grab it as an update in Eclipse.