[petrucci]

In terms of sites with large collections there is
cpdl.org The Choral Public Domain Library.

Many libraries share images of manuscripts.
http://www.bl.uk/reshelp/findhelpres...isedmusic.html

http://vc.lib.harvard.edu/vc/deliver...lection=scores

http://brbl-dl.library.yale.edu/vufind/

http://www.themorgan.org/music/

http://www.juilliardmanuscriptcollection.org/

https://urresearch.rochester.edu/vie...ollectionId=63

http://www.trentinocultura.net/porta...D=22652&mode=2

diamm.ac.uk (requires registration)