Python for .NET is a package that gives Python programmers nearly seamless integration with the .NET Common Language Runtime (CLR) and provides a powerful application scripting tool for .NET developers.
Tools and libraries to glue C/C++ APIs to high-level languages
C# scripting platform
A port of Clojure to the CLR, part of the Clojure project
ManagedCUDA aims an easy integration of NVidia's CUDA in .net applications written in C#, Visual Basic or any other .net language.
Java.Interop provides open-source bindings of Java's Java Native Interface (JNI) for use with .NET managed languages such as C#
Dynamic Language Runtime
IL2C - A translator for ECMA-335 CIL/MSIL to C language.