!IKVM is a virtual machine implementation which can run Java bytecode on .Net/Mono runtime.
!IKVM tool-set is very mature and it has been in development for years. 

ikvm.exe can run Java byte-code directly. It's just-in time compiler.[[BR]]
ikvmc.exe can translate Java byte-code to .Net !IL. It accepts .jar files and produces .Net assemblies. These assemblies use !IKVM.!GNU.Classpath.dll as implementation of Java libraries, which itself translated with ikvmc.exe.