This is the kind of software project that takes some serious craftsmanship to create, and that's exactly what the authors have put into it. Bravo!
My only real complaint is that the language has not stayed very stable, so some input files on the Mutopia site will no longer compile. There is apparently a conversion script for backward compatibility, but it's not well advertised. It would be nice to know how much one can depend on the language to stay the some from now on.
Prospective users who don't want to learn to type in music using the Lilypond language may want to check out the Rosegarden project, which is a GUI front-end for Lilypond.
An open, cross-platform journaling program.
A scientific plotting package.