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]

Java Virtual Machine. Works on many platforms and includes Just-In-Time (JIT) support for most of them. PersonalJava 1.1 compliant (but does not fully support the Java 2 Platform). [Open Source, GPL]

A free replacement for Sun's proprietary core Java class libraries. [Open Source, GPL with exception]

A Java virtual machine implementation that converts class files into C source files using the Soot Java bytecode analysis framework, compiles them with GCC, and loads them using a built-in ELF object file loader. [Open source, LGPL]

Provides free developer kits for creating and testing Java applets and applications on a number of platforms, including AIX and Linux. Also other Java tools. [Freeware]

Provider of embedded and realtime JVMs. Provides news and articles.

A former IBM research virtual machine focussing on compatibility, high performance, dependency analysis, and constructive assistance. Project information, FAQ, and links. [Open Source, CPL]

A free replacement for the "javax.*" classes (core and otherwise); sibling project to GNU Classpath. [Open Source, GPL/LGPL]

Enabling the integration of pure Java into Microsoft .NET applications. Tools support J2SE or J2EE, allowing .NET code to access J2EE facilities including EJBs, and JNDI. SE (Standard Edition) and EE (Enterprise Edition) versions.

Introduction to the Java Virtual Machine, links to JVMs for different platforms, and collection of related articles.