The Cat programming language is a functional stack-based programming language, with an optional static type-system. Think Forth meets Haskell. Cat is heavily inspired by the Joy programming language.
The main project is the C# source code for an interpreter of the Cat language maintained by the language designer Christopher Diggins. You can view the source code online at http://cat-language.googlecode.com/svn/trunk/.
The Cat interpreter is tested primarily on Windows but should also run on Linux and Mac using Mono.