http://www.youtube.com/watch?v=OnMphi-ZXmE#t=4m48s
Jimi Hendrix - Black Gold - new album demo tapes
(supposedly will be released by 2020)
Jimi Hendrix MLK tribute improvisation Newark, NJ - 5th April 1968, after learning of King's assassination.
Hendrix & Zappa jam at the Wreck Bar, The Castaway Hotel, Miami FL, May 20, 1968. Jimi jams on a Guild guitar with Noel Redding, Frank Zappa, Arthur Brown and Jimmy Carl Black (and according to one version, John Lee Hooker).
Zappa mimicking Hendrix on guitar:
http://www.sickthingsuk.co.uk/people/p-zappa.php
Jimi Hendrix sat in with The Mothers of Invention in the early 60's
Dinosaur Jr. demo tapes
Soundgarden early demo tape(s?):
http://grungebook.tumblr.com/image/9625448742
http://www.flickr.com/photos/fantagraphics/5549553524/
Tad's first Peel session - December 5th 1989:
http://www.bbc.co.uk/radio1/johnpeel/sessions/1980s/1989/Dec05tad/
any and all live bootlegs of Tales of Terror, Dickless, and Truly
…
Dinosaur Jr.