The websites listed below will allow you to download PDF files or JPEG images of
sheet music which is not under copyright. This usually means it was initially
published before the middle of 1923.
This list is hardy exhaustive, but it will get you started.
The first two sites on the list are actually common entry points for several libraries.
Be careful with the NYC Public Library URL, it is extremely long and must be typed on your browser address line EXACTLY as shown. The easiest way to save it is to click on it on this page and then save it to your "favorites" or "bookmarks" in your web browser.
Sheet Music Consortium by UCLA
http://digital.library.ucla.edu/sheetmusic/
IN Harmony: Sheet Music From Indiana
http://webapp1.dlib.indiana.edu/inharmony/welcome.do
Johns Hopkins University, Lester Levy Collection
http://levysheetmusic.mse.jhu.edu/
Duke University Libraries
http://library.duke.edu/digitalcollections/hasm/
Library Of Congress, Historic American Sheet Music
http://memory.loc.gov/ammem/award97/ncdhtml/hasmhome.html
University Of Oregon Libraries
http://oregondigital.org/digcol/sheetmusic/
Kirk Collection of Artist/Professional Sheet Music
http://odin.indstate.edu/about/units/rbsc/kirk/pop_titles.html
Kirk Collection of band Stock Arrangements
http://library.indstate.edu/about/units/rbsc/kirk/kirkdanc.html
Rare Ragtime Sheet Music
http://www.ragtimepiano.ca/rags/rare.htm
University Of Colorado Libraries
http://ucblibraries.colorado.edu/music/smp/
Detroit Pulic Library, E. Azalia Hackley Collection
http://www.thehackley.org
New York (City) Public Library
http://digitalgallery.nypl.org/nypldigital/dgkeysearchresult.cfm?parent_id=446131&word=music&s=1¬word=&d=&c=&f=&k=0&lWord=&lField=&sScope=&sLevel=&sLabel=&snum=0&imgs=20
University Of North Carolina Library
http://www.lib.unc.edu/music/
Band Music PDF Library
http://www.bandmusicpdf.org/
Christmas Carol Sheet Music
http://www.christmas-carol-music.org/Index_of_Titles.html
More lists of sources of sheet music:
IMSLP web site
http://imslp.org/wiki/IMSLP:Other_music_score_websites
Perfessor Bill web site (Getting a little old, some of the links are obsolete.)
http://perfessorbill.com/links.shtml