The most helpful and rich documentation for Android development can be found at http://developer.android.com/index.html. This is an official page for Android developers and it is a great reference whether you are beginner or experienced developer. It includes Android SDK Javadoc documentation as well as tutorials and design patterns.
Other good tutorials:
http://www.script-tutorials.com/10-attractive-android-tutorials-for-developers/ (This page includes 10 attractive Android tutorials links)
As we encounter new links, we will share them here. If you know other tutorials/links that you think it will be useful , please let us know.