http://www.nutz.org/ tab for Pop stuff
http://www.roughstock.com/cowpie/songs/ (tab) Cowpie for country
http://trillian.mit.edu/~jc/music/abc/FindTune.html JC's for Irish sheet music
http://www.numachi.com/~rickheit/dtrad/ sheet music for songs in the DT
http://levysheetmusic.mse.jhu.edu/ Levy for American after 1923(but it's down just now)
Feeling too lazy to do all those clickies, just copy and paste.

You'd be better off asking for specific tunes or songs, then we'd know where to send you. There are LOTS more out there, and I don't really understand what you mean by sheet music for guitar......tab, chords, what?