Although "free" is pushing it since it is basically the current DLC out there
http://freemusic.guitarhero.com/You have to enter the serial number from the guitar and the code from your manual.
"Rock and Roll Band" by Boston
"Hot Blooded" by Foreigner
"Jessie's Girl" by Rick Springfield
"No Rain" by Blind Melon
"Supernatural Superserious" by R.E.M
"The Shock of the Lightning" by Oasis
"Salute Your Solution" by The Raconteurs
"Human" by The Killers
"The Everlasting Gaze" by The Smashing Pumpkins
"Sliver" by Nirvana
You can only pick 5, apparently - but people are saying you can submit for 5, press back, then select the other 5