GNU Classpath - Mother of most of the VM

GNU Classpath provides core class libraries for use with virtual machines and compilers for the java programming language. GNU Classpath 1.0 will be fully compatible with the 1.1 and 1.2 API specifications, in addition to having significant compatibility with the 1.3, 1.4, 1.5 and 1.6 APIs.

It is the mother of most of the JVMs. Check out the list of other Virtual Machine products using GNU Classpath as its base. Intel and IBM are using GNU Classpath in their research