The nature of Android as Open Source project makes it easy for others to create useful libraries. This article lists popular open source libraries and useful example projects which makes it easier to create powerful Android projects.

https://github.com/bumptech/glide - Glide Image loading framework for Android

https://www.vogella.com/tutorials/AndroidButterknife/article.html - Android Butterknife, view injection framework

https://github.com/square/otto - Otto - Event Bus for Android

http://square.github.io/retrofit/ - Retrofit – A type-safe REST client for Android and Java

http://code.google.com/p/achartengine/ - AChartEngine, Charting Engine

http://ormlite.com/ - ormLight ORM mapper

http://greendao-orm.com/ - GreenDAO from Markus Junginger, ORM Mapper

https://github.com/google/Dagger - Dagger2 - dependency injection framework for Java and Android

http://code.google.com/p/android-scripting/ - Android Scripting, allows to run scripting languages on Android

The most important source of example code are the API demos of Android. You can create a project with this sample code via the Android Sample project wizard by selecting the API demos entry.

Roman Nurik from Google keeps this example repository. http://code.google.com/p/romannurik-code/ - Roman Nurik’s examples.

Mark Murthy (Android trainer, developer and book author) keeps his advanced examples in this repository. https://github.com/commonsguy/cw-advandroid - Mark Murthy’s advanced examples