LilyPond development is hosted at savannah.gnu.org/projects/lilypond

Here is an attempt at a simple explanation of the directory layout for
LilyPond's source files.

   Documentation/
     bibliography/		.bib files with references to books and articles
     windows/			building on windows
     pictures/			.xpms of the logo
     programmer/		programmers documentation, including a test document
     topdocs/			sources for the toplevel files (README.txt, INSTALL.txt, etc.)
     user/			User manuals
   buildscripts/		Scripts used during the build process
   debian/			Files for building .deb packages
   flower/			A generic library
     include/
   input/			Various input files
     test/			Test features: one file per feature
     bugs/			Show bugs
     no-notation/		Examples or bugs that do not produce output
     regression/		Various features for regression test
     tutorial/			Examples from the tutorial 
   lily/			Source code for LilyPond
     include/
   ly/				Standard include files.
   make/			LilyPond specific Make subroutine files
   mf/				MetaFont sources for the feta font
   midi2ly/			midi2ly source code
     include/
   mutopia/			Larger examples with full music pieces.
     E.Satie/
     F.Schubert/
     J.S.Bach/
   intl/			library for gettext
   po/				translations
   ports/                       access to mutopia archive
   ps/				postscript library files
   tex/				TeX library files
   scm/				Scheme initialisation and subroutine files
   scripts/			User scripts
   stepmake/			Generic make subroutine files