Great music few are aware of. Any genre.

Chicken Shack.  They are not well known but are pretty amazing late 60's/early 70's white blues from the U.K.
If you like jams you will like them.  No radio friendly material that I am aware of.  Even better...  

Muleskinner--the quintessential hippie bluegrass band.  They made one album and there is an audio recording available of a local PBS TV appearance (probably wasn't called PBS yet, but...)
Peter Rowan, Clarence White, David Grisman, Bill Keith and Richard Greene.  Man, what a band!

Yes! The Muleskinner album is fantastic, and that live TV appearance was put out on VHS tape. I don't know about DVD. Speaking of Peter Rowan, he joined Seatrain in time for their second album (s/t, produced by George Martin), and the version on that album of Lowell George's great song "Willin' " is my favorite.

Recent find for me is Steve Howell. Acoustic folk style blues, pre-war blues/jazz.

Here's a great one...

Jon Snodgrass  "Visitor's Band"