Tools and libraries Android programmers should know about, even if they don't use them.

Testing

1. Robolectric: A testing framework that provides much more functionality than Android's stock testing support and does not require an attached device.
2. AssertJ Android: Provides an intuitive fluent-style assertion syntax for Android (formerly FEST Android).
3. Espresso: A UI unit testing framework. Provides an API to interact with and assert things about UI elements. Supported by Google.
4. deckard-gradle: Sample project demonstrating how to use Robolectric to test and Gradle to build.

Persistence

1. SQLite Asset Helper: Simplifies the process of shipping with and updating SQLite databases.

Network

1. Robospice: Attempts to simplify the process of dealing with async tasks and network requests.
2. Android Async HTTP: Simplifies network requests by abstracting away direct interactions with the apache http client.

Build

1. Gradle Build System: The site for the Gradle build system, including usage and release notes.

Code

1. Android Annotations: Heavy use of annotations in an effort to make code more readable and maintainable by abstracting away some complexities of Android development.
2. Butter Knife: Uses annotations to inject and manage views.
3. AppIntro: Create intros to your app.
4. Material-Icon-Lib: A repository of material design icons featuring strong integration with Android Studio.