The current stable release is version "r20" of the SDK Tools and ADT 20.0.3. You can download the official bits from http://developer.android.com/sdk/index.html.
We are currently developing r21 and ADT 21 in the AOSP git trunk.
If you want to build it yourself, follow the instructions in the Build Overview document. Recent user-visible changes to the trunk are listed in the Recent Changes document.
For Studio preview channels, see the Android Studio Downloads page.
July 24, 2014: Android Studio 0.8.4 Canary Build Released. See the Canary channel for more.
June 27, 2014: Android Studio Beta Released. See the Beta Channel for more.
April 28, 2014: Eclipse ADT 22.6.3 and SDK Tools 22.6.3 Released.
See http://developer.android.com/tools/sdk/tools-notes.html and http://developer.android.com/tools/sdk/eclipse-adt.html
For more information on the status of each of our branches, see the Release Status page.