Operating system for mobile devices. Developed by Google and the Open Handset Alliance.
Contains the SDK downloads, documentation, class reference and tutorials. Start here.
Android Developers Google Group
Alternative developer discussion forum for Android.
Android Open Source Project (a.k.a. AOSP)
Contains all necessary information about the Android source code.
Android page on Wikipedia
Detailed information about the Android OS.
General information about the Android OS.
Google I/O developer conference
Contains detailed videos and slides by Android product engineers
For non-developer questions, see the Android Enthusiasts StackExchange.