Music programming language.
Velato is a programming language, created by Daniel Temkin in 2009, 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.
Official document: http://velato.net/
Download compiler: http://velato.net/Content/Velato/Velato_0_1.zip