The Mutopoia Project
Free sheet music! Seriously. No crap. It’s like the Project Gutenberg for sheet music.
You just know it’s run by nerds: “All music is available as Postscript (.ps) and PDF (.pdf) files, for both A4 and Letter paper sizes, as well as Lilypond’s own file format (.ly).”
That and the site is barely navigable, a sure sign of true nerd-dom.