J.A.D.E. Java Addition to Default Environment
The JADE library provides "Context Programming".

jga: Generic Algorithms for Java
The goal of this project is to provide functionality roughly analogous to the portions of the C++ STL library that are not already provided by java.

Jodd
Jodd is a generic purpose open-source Java library. It helps in development of Java applications, both stand-alone and server-side. You may think of Jodd as swiss army knife for Java: small, sharp and with plenty handy gadgets included.

Kataba Dynamics
Set of core libraries for Java, inspired by dynamic languages like Ruby and Python.

NailGun
NailGun is a client, protocol, and server for running Java programs from the command line without incurring the JVM startup overhead. Programs run in the server (which is implemented in Java), and are triggered by the client (written in C), which handles all I/O.

Ostermiller Utils
Libraries for common tasks such as CSV, Base64, Circular Buffers, MD5, and Significant Figures.

Thought River Commons
The Thought River Commons is a collection of utilities and other nifty code items developed by Thought River North and Thought River South for use in our various internal projects, and made available here for public use. Most experienced developers know that over time, the same needs are often encountered again and again. We wrote the Thought River Commons to satisfy those sorts of needs, and we figured that some other developers out there might make use of it, as well.