A metaprogramming platform, including a web front end. Allows it users to extend existing programming languages (currently C#). Create new language contructs, new types of types, or entire semantical libraries. In fact, baring the backbone of the language (i.e. expressions, traditional statatements, etc), entire programming languages could be developed. Without the need for doing any heavy lifting.

The platforn is at its core a substitution engine, which means that instead of compilation, the user performs translation from one language to another. With the obvious advantages of being able to reuse the host language's libraries, code generation, etc. As results, extensions developed in Excess tend to be very solid. Once it translates well it will always translate well. And if the host language compiles it, it will be correct.

Start at the wiki, which contains the documentation and plenty of samples. Including xs, a complete language previously developed by the author which was implemented in about 600 lines of code this time around.