Following the update to Android 4.2 (still called Jelly Bean) the SDK and the SDK Tools have been updated.
The update to the SDK supports all the new features in the decimal point upgrade to Android. The biggest changes from the developer's point of view is the ability to target individual external displays. Rather than just being able to mirror the main screen, apps can now target content to any connected display. Another improvement is the moving of renderscript computation to the GPU - but only on the Nexus 10.
From the user's point of view the major change is the ability to run widgets on the lock screen. For example you can now view your gmail inbox, the clock or a media player on the lock screen. To take advantage of this facility, widgets have to be updated but the change is a small one. Google clearly thinks that the ability to place widgets on the lock screen is an important one:
"You can create interactive daydreams that users display in this mode, and they can include any type of content."
The new Android SDK Tools has a multi-config editor which lets you see your UI design on a range of different screen configurations. This is such an obvious time saver that you just have to say "why didn't I invent that".
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There are also more app templates and a UI Automator test framework. The UI test framework lets you automate testing tasks on physical devices. It only works with Android 4.1 and later.
Other minor, but welcome, improvements include a one-click SDK installer. This will make many books that describe downloading Eclipse, then the SDK, then the ADT and so on out of date, but it makes getting started on a new machine easier. Now you can download everything you need with a single click.