From time to time people ask for "lead sheets" for popular tunes out of copyright so they can arrange them. In the process of carving that spiritual and secular Christmas arrangement/track list out of my 2-meg list (see http://www.mjbarkl.com/bshop.htm) I found several sites with thousands of old song sheets archived and available for downloading:

Library of Congress - http://memory.loc.gov/ammem/smhtml/smhome.html

    Music for the Nation: American Sheet Music, 1870-1885 consists of

http://memory.loc.gov/ammem/browse/ListSome.php?category=Performing%20Arts,%20Music

    which indexes LOC and other sites, including Duke U.  - the LOC

Library and Archives Canada - http://www.collectionscanada.ca/sheetmusic/index-e.html

    . . .grown to become the most comprehensive in the country [Canada],

Duke University - http://scriptorium.lib.duke.edu/sheetmusic/

    This is a collection of over 16,000 pages of sheet music from 3,042

Johns Hopkins University - http://levysheetmusic.mse.jhu.edu/advancedsearch.html

    The Lester S. Levy Collection of Sheet Music is part of Special

Indiana University - http://www.letrs.indiana.edu/web/s/sheetmusic/

    [Different from?]

Indiana State University - http://library.indstate.edu/level1.dir/cml/rbsc/kirk/kirk-idx.html -

    Amassed by C. Weir Kirk, a Terre Haute, Indiana area musician, the Kirk

University of Colorado - http://ucblibraries.colorado.edu/cgi-bin/sheetmusic.pl?all&titles&1#top

    Digital Sheet Music Collection

Also interesting/useful:

SPEBSQSA downloadable sheet music - http://stellent.spebsqsa.org/web/groups/public/documents/webassets/id_036338.hcst#freeHarmonet files - http://launch.groups.yahoo.com/group/bbshop/files/

Various, less useful to barbershop arrangers but with a bit of spiritual:

http://www.mutopiaproject.org/piece-list.html - All music in the Mutopia

    Project is free to download, print out, perform and distribute. There

http://www.cipoo.net/music_a.html (Choral) and http://www.cipoo.net/music_v.html

    (voice & piano) - The download area contains now 873 files

http://directory.google.com/Top/Arts/Music/Resources/Downloadable_Music_Sheets/Public_Domain/

    Less useful than the list you are now reading - I'm surprised at its gaps.

http://www.freesheetmusic.net/Choral.html - many links, but I had a hard time

    finding any sheet music.

sheet music public domain - search for these 4 words on Google

    yields 380,000 hits, but most of them are useless

Classical?: http://www.sheetmusicarchive.net/ - the Sheet Music Archive - Free public

    domain editions of public domain classical music

http://www.cpdl.org/ - The Choral Public Domain Library (CPDL) is the

    largest website devoted exclusively to free choral sheet music. Begun in

http://icking-music-archive.org/ - A continuation of the GMD Music Archive

    [a ton of classical sheet]

There are others, some of which require Sebelius music software.

The biggest problem I had with all this is that it's poorly indexed and not centrally indexed: some sites are fragmented, some have only search functions and not browse, Google doesn't seem to list all the songs, recent pop hits of the same name bury the older ones in Google output, and so on, and Google results on many specific songs in these collections is somewhat haphazard. Some lead to arc.cs.odu.edu (Arc is an experimental research service of Digital Library Research group at Old Dominion University) cross-references that I was unable to decipher until I tried going to arc.cs.odu.edu itself and re-entering my query.