Folk music is always worth checking out. Knowing a few standards can make busking or jamming a breeze, if you're that way inclined. Anyway, get folked!

Various - Acoustic Classic - 44 Songs Songs Of The 60s & 70s
http://www.hitsquad.com/smm/sheetmusic/titles/39.shtml

Various - Golden Encyclopedia Of Folk Music
http://www.hitsquad.com/smm/sheetmusic/titles/354.shtml

Simon And Garfunkel - Great hits - Easy Guitar
http://www.hitsquad.com/smm/sheetmusic/titles/38.shtml

Bob Dylan - Bob Dylan Made Easy For Guitar
http://www.hitsquad.com/smm/sheetmusic/titles/51.shtml

John Denver - John Denver Anthology
http://www.hitsquad.com/smm/sheetmusic/titles/37.shtml

Check out the range of sheet music titles at:
http://www.hitsquad.com/smm/sheetmusic/