Caffeine

Caffeine is a free high-performing interoperability solution between the Java platform and the .NET framework. Caffeine’s key differentiator is that does not replace the Java Virtual Machine. Instead, your JVM of choice is hosted within the .NET runtime, ensuring that vendor-specific VM optimizations are preserved. The specific objectives of Caffeine are:

Run on any combination of .NET runtime (1.0+) and Java Virtual Machine (1.2+).
Allow full reuse of any Java library from a .NET environment, working exclusively at the API level and avoiding bytecode translation of the actual implementation.
Provide optimal performance, running the JVM and .NET under the same process and avoiding network or IPC costs.

Please check the Caffeine project on Berlios, the architecture guide, and the introduction to runtime interoperability.