A open sourcejava card virtual machine implementation. And also some part of the VM code can be used as part of kinds of tools such as javacard bytecode disassembler.
jbytecode is a Java bytecode disassembler/assembler written in Python. Dissasembly code is aligned with Java bytecodes in the class file so modification and re-assembly is always possible, even when class is obfuscated.
JTools.Ruby is easy way to access Java 2 class-files via scripts written in Ruby language. There are class-file disassembler, assembler and simple Ruby to Java bytecode compiler. The work is at the start point.