The Mutopia Project is the Project Gutenberg equivalent for music!
It’s library isn’t as extensive, but it does have over 2000 pieces of music available for use, in both PDF and MIDI files. The MIDI file part seems to be fairly unique, and lets you access and use the music for creative purposes beyond just playing.
There are some other similar resources for sheet music as well:
The Petrucci Music Library (known as IMSLP)
IMSLP has about 127,000 scores across almost 16,000 composers – all public domain.
Gutenberg’s sheet music section hasn’t been added to since about 2006, but it does still have quite a few works. Probably nothing here you couldn’t find above, but worth mentioning all the same. They made a conscious decision to stop pursuing this part of their library due to other services, like IMSLP having stronger workflow due to specializing on just music, as opposed to text and music.
About 28,000 choral and vocal works by about 3,000 composers. Seems like mostly church stuff – vocals and organs, for all the obvious reasons.
I would have killed for services like these growing up when I was playing a lot of music and struggling to find new things to play / challenge myself with. Anyone with a passion for playing or writing music will probably find something worthwhile with a bit of time spent poking around these sites.