Jimmy Witherspoon
"Live at the Monterey Jazz Festival" (Vinyl)

Recorded live in 1960, this shows a very jazzy Witherspoon bluesing it up with the likes of Roy Eldridge, Ben Webster, Woody Herman etc.
There was no rehearsal for this set - according to story Jimmy Witherspoon walked on stage with the musicians who had played before him and shouted "Down Home in A-flat!" and away they went.

It's pretty cool to have this sort of performance on record like this.