Here is a list of my favorite sources for free music scores:
http://www.free-music-scores.com/index.htm music by hornplayer and arranger Jean-François Taillard
http://www.free-scores.com/index_uk.php3 a large selection of free scores
http://www.music-scores.com/ some 300 scores, for members
http://www.mutopiaproject.org/ the Mautopia free music project
http://imslp.org/ the International Music Score Library Project (IMSLP)!
http://www.rowy.net/sheet_music.html mainly for solo instruments
http://www.classiccat.net/ the free classical catalog
http://www.mfiles.co.uk a rich music site with music sheets, midi, mp3, and more