Lilypond is a music typesetting program - it takes a musical score in text format, and turns it into pretty pictures :-) Here you will find some of the stuff I have done with Lilypond (though as of writing [April 2005], it's rather bare as I've only been using it two days :-) ).
Each folder has a number of files in it - the file ending .ily (or .ly) is the original source file for Lilypond, and the supplied Makefile allows you to create the actual score by typing 'make' at a command prompt. The rest of the files are created by Lilypond: