A open source java 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.