Begun in December 1998, the Choral Public Domain Library is one of the world’s largest free sheet music sites. You can use CPDL to find scores, texts, translations, and information about composers.
While the site concentrates on sheet music, quite a number of works also have corresponding MIDI files on their pages. And that can be a gold mine for remixers with midi capable software. The site contains classics from giants of choral music including Bach, Handel and Mozart, but also works by contemporary composers like Leanne Daharja Veitch, who reminded me of this most excellent musical resource and writes “I’m encouraging all small-time composers (like me) to make our work creative commons, and accessible to all.”