The advantage of this compiler over Sun's is its ability to generate native machine code - so if you are not coding for the web and simply want to use Java as a programming language, this might be a good compiler to check out. IBM's Jikes Jikes is IBM's Java compiler that is supposedly faster than most other compilers. It is also able to compute the dependency requirements in programs and so generate makefiles. If you want faster development cycles, you might also fancy its ability to perform incremental compilations. The compiler handles constructs exactly according to the Java specifications.
Source code appears to be provided.
You can get free Java Development Kits for a number of platforms from the creator of Java. The kits include the compiler, debugger and libraries for generating Java bytecode. It's is totally free as well as it's documentation is free also. JDK is a complete development tool and compiler for Java programmers.