We've already mentioned that the open design of .NET actively encourages the use of other languages, and Microsoft is keen for this to happen. At the time of writing, the following languages were also becoming available:
Dyalog APL/W Version 9.
Fujitsu COBOL.
Component Pascal.
Eiffel.
Haskell.
Mercury.
Oberon.
Perl.
Python.
Scheme.
Standard ML.
There are others in the pipeline, and the best resource is http://www.gotdotnet.com, which contains an upto-date list of languages.
If you're into building your own languages, the SDK comes with some good sample compilers that show you how to do this. They are in the Tool Developers Guide. There's also plenty of documentation about assemblies, IL, and so on.