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] http://gcc.gnu.org/java/
Extending the Mozilla plug-in architecture to allow Java virtual machines to be plugged into Mozilla-based browser releases. [Open Source, MPL] http://www.mozilla.org/oji/
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] http://www.kaffe.org/
A free replacement for the "javax.*" classes (core and otherwise); sibling project to GNU Classpath. [Open Source, GPL/LGPL] http://www.gnu.org/software/classpathx/
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] http://www-106.ibm.com/developerworks/java/jdk/
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] http://jcvm.sourceforge.net/