I just stumbled upon this site called Mutopia Project. Judging by the name, it seems to be about the construction of a utopia for mutants. Or maybe that's just me. But instead my guess is that's its Music Utopia... cleveeer. But it is a really cool project, its like the Project Gutenberg of the sheet music world.
All of those famous dead white men that composed all that music that we are all supposed to know... well, they are long dead, that means their music is out of copyright and very safely in the public domain. That doesn't mean you can scan some book of sheet music from one of those dead white men and distribute it freely because that publisher has a copyright on that book. These guys have made sure that they started with sufficiently old enough copies so that they really are in the public domain.
Now seeing that an Austrian university is a major contributor, there will be lots of music from dead, white Germanic men. But there might be a lot of Welsh music too, since the official project languages are: English, Deutsch, Français, and Cymraeg.
Let's hope the music of lots of other people get added to this too.