Fleetwood Mac in the Peter Green days (aka "before the chicks"). Actually, Christine McVie (nee Perfect) supposedly played piano on this album, even though she's uncredited (as a session musician).
It's got this weird little pause in the beginning of the song, at about the 1:06 mark. Also, it starts out like a jam already in progress; like we're only getting part of the song (even though it's like 7 minutes long). This is from the 1969 release "Then Play On," which actually had 2 releases. I've never heard the album, even though I like a lot of their early stuff, so the song may have actually started on the previous track.