Retlang and Jetlang are open-source libraries for the .NET CLR and JVM that provide concurrency through in-process messaging. Mike Rettig, one of my colleagues at DRW, is the lead of both of these projects.
Today at Speakerconf I presented on these libraries, and the slides can be found here (in Keynote ’09) and here (in PDF).
I encourage you to use the project mailing lists (for Retlang and Jetlang) if you are interested in learning more.
Mike Roberts