There was some flap last week when the International Mozart Museum purchased and made copies of the entire Neue Mozart-Ausgabe (complete works of Mozart) available online. As it turns out the license is quite restrictive and only solo studying is allowed. Note that this work (as a farily new translation) is still in copyright, older public domain versions are already available.
In any event it set me off on a quick search around the internet for public domain sheet music. I didn’t come up with much, certainly nothing on the scale of Project Gutenberg. Mutopia is about as close as you can come, but even this is quite a small project. Project Gutenberg and others do have some music available, unfortunatly a lot of it is only available in proprietary formats requiring expensive software to view or print (more on this below). If all you’re looking for is scans of pages you can try sites like these but it would be nice to have it in a truly digital and free format.
From what I can tell you really need to make things available in multiple formats, preferably LilyPond which does a subpurb job of old style engraving and outputting into PDF (for people who want to print it and play it), these people are serious about making music look the way it should and not computer generated sheet music which generally looks quite poor. Being an open format converters can easily be written to convert LilyPond files into other formats.
I think you’d be interested in the International Music Score Library Project (IMSLP), which just recently passed 4000 (!) scans of scores, nearly all of them in the public domain. http://www.imslp.org/