Applet Servlet: Free online compiler and runner for Java applets.
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.
DJ: Java decompiler and editor for Windows. [Freeware]
Jasmin: A Java bytecode assembler. [Open Source, GPL]
Java(tm) Bytecode Assembler: Free, but for research and educational use only.
Jikes: IBM Research Jikes Compiler Project. A Java compiler written in C++. [Open Source, IBM Public License]
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)
The KOPI Project: Java compiler, byte-code assembler, disassembler. [Open Source, GPL]