osmdroid is a (almost) full/free replacement for Android's MapView (v1 API) class. It also includes a modular tile provider system with support for numerous online and offline tile sources and overlay support with built-in overlays for plotting icons, tracking location, and drawing shapes.
Spock is a testing and specification framework for Java and Groovy applications. What makes it stand out from the crowd is its beautiful and highly expressive specification language. Thanks to its JUnit runner, Spock is compatible with most IDEs, build tools, and continuous integration servers.
Gradle Script Kotlin provides support for writing Gradle build scripts using JetBrains' Kotlin language. It aims to provide Gradle users with a rich, flexible and statically-typed approach to developing build logic in conjunction with the best IDE and tooling experience possible.
RxJava is a Java VM implementation of Reactive Extensions: a library for composing asynchronous and event-based programs by using observable sequences.
It extends the observer pattern to support sequences of data/events and adds operators that allow you to compose sequences together declaratively while abstracting away concerns about things like low-level threading, synchronization, thread-safety and concurrent data structures.
Android-Retainable-Tasks is an easy to use mini-library for easy asynchronous background tasking with callback support to the UI. This library is based on the Android
AsyncTask implementation but with support for retaining tasks and therefore surviving configuration changes (orientation).
Thrifty is an implementation of the Apache Thrift software stack for Android.
Thrift is a widely-used cross-language service-definition software stack, with a nifty interface definition language from which to generate types and RPC implementations. Unfortunately for Android devs, the canonical implementation generates very verbose and method-heavy Java code, in a manner that is not very Proguard-friendly.