RxJava – Reactive Extensions for the JVM – a library for composing asynchronous and event-based programs using observable sequences for the Java VM.

Apache Druid: a high performance real-time analytics database.

A damn simple library for building production-ready RESTful web services.

Source for the TechEmpower Framework Benchmarks project

The Physical Web: walk up and use anything

Simple OAuth library for Java

Facebook's Realtime Distributed FS based on Apache Hadoop 0.20-append

(ABANDONED) Simple stock trading system that optimizes its parameters with particle swarm optimization

Java client and server implementation of Redis

A simple tool that finds serious bugs in Java exception handler

Statebox implementation for the JVM