Free Java Packages
JGraphT is a free Java graph library that provides mathematical graph-theory objects and algorithms.
JSON.simple is a simple Java toolkit for JSON. You can use JSON.simple to encode or decode JSON text.
An application framework for Java, Spring solves core enterprise development and runtime problems, offering configuration via Dependency Injection; declarative services via AOP; and packaged enterprise services. Developed by SpringSource and released under an Apache license.
Universal Java Matrix Package
The Universal Java Matrix Package (UJMP) is a Java library which provides implementations for sparse and dense matrices, as well as linear algebra calculations such as matrix decomposition, inverse, multiply, mean, correlation, standard deviation, etc.
International Components for Unicode is a portable set of IBM Unicode globalization libraries for C/C++ and Java. ICU includes support for locale sensitive string comparison; date, time, number, currency, and message formatting; character set conversion; and additional features. It is avaliable under the X license, which is apparently compatible with the GPL license, although not copyleft. See the website for more details.
BBN Technologies OpenMap package is an Open Source JavaBeans based programmers toolkit. OpenMap provides the means to allow users to see and manipulate geospatial information.
JavaBeans Activation Framework (JAF)
With the JavaBeans Activation Framework standard extension, developers who use Java technology can take advantage of standard services to determine the type of an arbitrary piece of data, encapsulate access to it, discover the operations available on it,
JavaHelp software is a full-featured, platform-independent, extensible help system that enables developers and authors to incorporate online help in applets, components, applications, operating systems, and devices.
The JavaMail API provides a set of abstract classes that model a mail system. The API provides a platform independent and protocol independent framework to build Java technology-based mail and messaging applications. The JavaMail API is implemented as a Java platform optional package and is also available as part of the Java 2 platform, Enterprise Edition. Sun provides a royalty-free reference implementation, in binary form, that developers will be able to use and ship.
SNMP4J is an enterprise class free open source and state-of-the-art SNMP implementation for Java™ 2SE 1.4 or later. SNMP4J supports command generation (managers) as well as command responding (agents). Its clean object oriented design is inspired by SNMP++, which is a well-known SNMPv1/v2c/v3 API for C++
Kawa, the Java-based Scheme system
Kawa is a Scheme environment, written in Java, and that compiles Scheme code into Java byte-codes.
The Mauve Project is collaborative project whose goal is to create a Free suite of functional, black box, tests for the core Java libraries.
GNU Classpath (prerelease source available for download under GPL)
GNU Classpath is a set of essential libraries for supporting the Java language. Our goal is to provide a free replacement for Sun's proprietary class libraries so that the GNU system can run programs written in the Java language.