Free sheet music collections

There are a number of sites which collect public domain sheet music on the web. Here's a few which I've found useful, or which look interesting.

• IMSLP Large, mostly scans of old commercial printings. They seem quite careful about documenting the public domain status of things.
  
• Digitized Scores from the Sibley Library at Eastman. If they've got it on paper, and it's out of copyright, they'll scan it for you too.
  
• Mutopia An archive for music typeset with LilyPond. Either CC or public domain.
  
• NY Phil Digital Archives You can't print or perform from these, but you do get to look at, say, Leonard Berstein's markings on the Vivaldi Bb concerto
  
• Tuben A selection of early music, handset in TeX by one guy.
  
• Werner Icking Music Archive Also see their list of other music archives
  
• Digital Mozart Edition Online version of the definitive (?) NMA edition of Mozart's works.
• Danish National Digital Sheet Music Archive Reicha quintets, Nielsen, lots of other things too
  

Updated The NY Phil archive is kind of addictive. Check out the bumper sticker Leonard Bernstein stuck to the first page of Mahler 6, something I'd heard of before. There's also a score of Mahler 1, marked up by Mahler, and no doubt zillions of other things besides.