1) The Grateful Dead
By marrying folk, bluegrass, country, blues, rock 'n' roll and jazz to what we now call "psychedelic" music, the Dead laid the template for all searching, inquisitive rock musicians. Ensemble improvisation in rock did not really exist before the Dead started doing it in the late '60s.
2) Tie: The Allman Brothers Band, Bob Dylan's Band (1999-2005)
The blues and jazz meet a distinctly southern take on rock 'n' roll with the Allmans, the band that has given us some of the most soulful guitar playing of all time.
Bob Dylan led his strongest ensemble of improvisers with the help of guitarists Larry Campbell, Charlie Sexton and, for a brief period, Freddie Koella, during a five-year period of artistic rebirth.
3) King Crimson
No matter what the incarnation of this Robert Fripp-led ensemble - late '60s grandiosity, '70s exploratory avant garde, '80s Adrian Belew-led twisted prog-pop genius, and the dark, subterranean industrial rumblings of the '90s and beyond - King Crimson has pushed the envelope. Hear the band's influence all over the contemporary jam scene.
This German improvisation ensemble brought indelible grooves to the prog-rock table in the '70s. Bizarre, brilliant, still way ahead of its time.
5) Frank Zappa
A wholly singular guitarist, who may well be the greatest rock-based improviser we've yet seen, Zappa balanced his meticulously arranged and painstakingly composed pieces with extremely heady in-concert improvisations. College theses have been written about the complexity of Zappa's stuff, but mostly, it balanced incredible technique with much heart and humor.
The birth of the blues-rock power trio. Still a huge influence on any improvising guitarist, bassist or drummer.
7) Miles Davis
Davis' late-'60s and '70s bands made a wholly unique improvisational noise, normally over relatively static tonal centers and mantralike rhythmic motifs. Like music from another planet, or somewhere in the future on this one. This stuff opened the door to jazz for countless rock musicians aspiring to expand their craft.
8) Jimi Hendrix's Band of Gypsies
The blues run through the molecule-shifting prism of Hendrix's imagination. Still the standard to which all others attempt to rise.
Bizarre, ambitious, way out there. At its best, Phish was the finest jam band of its generation.
10) Led Zeppelin
Blamed for birthing heavy metal - ironically, a music marked by very little improvisation and even less of the blues - Zeppelin was in fact a progressive band steeped in electric blues, but willing to take on everything from Middle Eastern music to folk and country. Live, the band improvised all over the joint.