IronPython
I spent some time yesterday test driving IronPython. Jim Hugunin has posted a simple tutorial on his blog demonstrating how to use IronPython to interactively build a simple WinForm app. It is quite amazing how familiar the code looks due to the use of the .NET Framework class libraries. It is great to be able to do .NET prototyping and exploration while taking advantage of the syntactical niceties of Python.
All in all, the IronPython distributable is 380kb, and the core assemblies are about 300kb. It is quite surprising how tiny the implementation of a language on top of the CLR can be.