The Project Gutenberg of printed music. An archive of music whose copyrights have expired, leaving them in the public domain. The music is input by volunteers and typeset by GNU Lilypond.
I've contributed to the archive; it's fun and challenging to do, and it's rewarding to see the archive grow. It's fairly small right now, with a few Bach preludes and fugues and various other odds and ends, but it's growing. I don't expect it or Project Gutenburg to ever completely replace their printed counterparts, because sometimes the nice cover and binding are worth the money, but it's great for when you just want to check something out, or learn just one piece from a collection.
It's important to ensure that any editions made to the music have had their copyrights expire before you enter them. A common way around this problem is to input from an urtext, or non-edited, score.