Article by David K. Friedman and David A. Wheeler. It briefly introduces Java, provides a bit of Java history, and describes various Java implementations.
Sun Microsystems backed industry body, Java Community Process (JCP) oversees development of platform and language, voted to alter community structure, officially supporting OSS implementations of Java, JCP 2.5. [The Register]
The GNU Compiler for Java - compiles Java source or bytecode to native machine code. Most APIs are supported, except the AWT and Swing. [Open Source, GPL]
A former IBM research virtual machine focussing on compatibility, high performance, dependency analysis, and constructive assistance. Project information, FAQ, and links. [Open Source, CPL]