I've also found all the early Pink Floyd albums, in their entirety. Syd Barrett was there for most of these. Piper At The Gates of Dawn, A Saucerful of Secrets, Ummugumma, etc. It's really interesting music, music which you never hear on the "classic" rock stations.
I always like to see unlikely or uncommon unions between musicians. I discovered recently that Carlos Santana did an album with Alice Coltrane, John Coltrane's wife, in 1974. Also Jerry Garcia worked with Ruben Blades, the band leader from Panama.
This is just a sample of the stuff that you can find on the internet that is out of the ordinary. I'm sure there is much more than what I've seen. If you have any other stuff of interest, post it on the comments.