Public domain, pdf files, and the maintainer encourages contributors to include source (and sound or at least midi) files. Most of my music <
http://icking-music-archive.org/ByComposer/Eijkhout.php> has lilypond (or occasionally sibelius) source files. This site is very much alive, with several contributions per day.
The maintainer of this archive is getting on in age and has health concerns, so there is a discussion going on about the future of the site. Maybe disapointed mutopia enthousiasts can play a role here?