Gotta tell ya about PERRO - the Planet Earth Rock & Roll Orchestra:

The Grateful Dead, Jefferson Airplane, Crosby Stills Nash, Quicksilver Messenger Service, and a couple of other bands were all friends in the late 1960's. The bands shared bills at all of the major music venues and were socially friends, as well.

This, of course, led to many impromptu jam sessions and weird one-offs.

The one I linked to above is Jerry Garcia, David Crosby, Jorma Kaukonen, Phil Lesh, and Jack Casady, hanging out and jamming in a studio together on January 13th and 14th, 1971. It's just a jam session; not an official "PERRO" recording. It had been rumored to exist, but nobody had heard it until a couple of months ago, when the guy who had been holding the tapes for decades, uploaded this 32 minute recording to YouTube.

This is, to put it simply, the best of the best at the time, enjoying each other's company, hanging out, and having a fun jam session. The "tracks" are loose and some have no definite beginning or ending... but they exude this deep essence of the Summer of Love and the music of the San Francisco at a truly unique time in history.