Velato is a programming language which uses MIDI files as source code: the pattern of notes determines commands. Velato offers an unusual challenge to programmer-musicians: to compose a musical piece that, in addition to expressing their aims musically, fills the constraints necessary to compile to a working Velato program. Each song has a secret message: the program it determines when compiled as Velato.
Velato on Esolangs.org
A three-part introduction to the language on my blog:
Download the compiler
Create Digital Music's coverage of Velato
Velato is currently Windows-only, and requires the .NET Framework 3.5 to run.
This is freeware, do whatever you want with it; just don't sell it.
This download includes the compiler itself, and Stephen Toub's MIDI library, used to read the .mid files.