A free MSIL -> Java Bytecode compiler available at www.mainsoft.com 
. Enables any .Net app to port to Linux/Tom Cat/Postgre Sql without code alteration. The enterprise version gives complete support for integrating .Net and J2EE apps at a source code level.
Depends to some extent on Mono, but that's a matter of convenience. Grass Hopper represents a radical change in the Great Game because now .Net apps can be hosted in any bean container and Java libraries can be linked in without any Web Services junk in between. Will Microsoft survive without a lock on the OS? Will Java survive now all the .Net languages can play in the JVM? By the pricking of my thumbs ...
Long before it was some no name compiler, Grasshopper was an important OS research project aimed at creating the minimal abstractions necessary to support Transparent Persistence.
See original on c2.com