I've never heard of most of the bands listed in this thread. I'll have to seek them out. Part of my definition of ground-breaking is not only doing something new and different, but also having a strong influence on lots of other people, so to name a few that come quickly to mind and in no particular order:

The Beatles
Led Zeppelin
Bothy Band
Jethro Tull
Gryphon
Martin Carthy
Fairport Convention
Malicorne
Crosby, Stills and Nash
Steeleye Span
Jimi Hendrix
King Crimson
Yes
Albion Band