Compiling the Java Programming Language
   Issues related to compiling Java into native (object) code. Treats why, compiler architecture, links (Java compiling, compiling generally), frontend and infrastructure software.
   http://www.bearcave.com/software/java/java_compiler.html

Jasmin
   A Java bytecode assembler. [Open Source, GPL]
   http://jasmin.sourceforge.net/

Java(tm) Bytecode Assembler
   Free, but for research and educational use only.
   http://tinf2.vub.ac.be/~dvermeir/courses/compilers/javaa/

Languages for the Java VM
   A list of tools for programming to the Java virtual machine using a language other than Java itself. (Robert Tolksdorf)
   http://www.robert-tolksdorf.de/vmlanguages.html