There are several reasons that might send someone looking for free sheet music:
• when you need a piece for research or analysis
• when you need a piece for performance
• when you want to create your own arrangement of a piece
• when you can’t remember how a certain piece sounds
Here are some of the places on the web worth checking for large collections with free access. Note that viewing some scores may require additional software, for example Adobe Reader or Scorch.
For a wide range of music:
• Sheet Music Consortium is a group of organizations working to create an accessible collection of digitized sheet music. Collections, most of which are substantial in their own right, include:
    • Library of Congress
    • Duke University
    • National Library of Australia
    • The Maine Music Box
    • Indiana University
    • UCLA Archive of Popular American Music
    • John Hopkins University Levy Collection
• IMSLP / Petrucci Music Library has over 15,000 works
• SibeliusMusic.com sports a new look (as of 5/8/09) and over 90,000 scores. Some scores have a cost, but adding “free” to your search will help sort them.
For choral music specifically:
• The Choral Public Domain Library has over 10,000 scores.
• Cipoo.net: Public Domain Choral Music is approaching 2,000 scores.