Sebastian will be a symbolic music analysis and composition library written in Python.
I partly think of it as being a "Mathematica for music". By symbolic, I mean that it will focus more on the sorts of abstractions made by music notation and theories of harmony and counterpoint than lower level numerical operations that one might use in, say, music synthesis.
I hope it to be very practical as an analysis tool for existing compositions (especially from the common practice period) and also a tool for computer-assisted composition. It will also be a bit of a test bed for ideas I've had for a while around applying linguistics and abstract algebra to music.
See http://github.com/jtauber/sebastian for code so far.