Following are the must know and must use libraries for Android developers. Any other library suggestions are welcomed and will be added below.
Retrofit: A type-safe HTTP client. http://square.github.io/retrofit/
Picasso: A powerful image downloading and caching library. http://square.github.io/picasso/
ActiveAndroid: Active record style ORM library. http://www.activeandroid.com
OKHttp: An HTTP & HTTP/2 client. http://square.github.io/okhttp/
EventBus: Android optimized event bus that simplifies communication between Activities, Fragments, Threads, Services, etc. http://greenrobot.github.io/EventBus/
Hugo: Annotation-triggered method call logging for your debug builds.. https://github.com/JakeWharton/hugo
ButterKnife: Bind Android views and callbacks to fields and methods. https://github.com/JakeWharton/butterknife
GSON: JA Java serialization/deserialization library to convert Java Objects into JSON and back. https://github.com/google/gson
Tape: A lightning fast, transactional, file-based FIFO . http://square.github.io/tape/
Crashlytics: lightest weight crash reporting solution. https://fabric.io/kits/ios/crashlytics/