"Alton Kelley, whose psychedelic concert posters for artists like the Grateful Dead, Jimi Hendrix, and Big Brother and the Holding Company helped define the visual style of the 1960s counterculture, died on Sunday at his home in Petaluma, Calif. He was 67."
http://www.nytimes.com/2008/06/04/arts/design/04kelley.html?_r=1&oref=slogin
Some examples of Kelley's work can be found here:
http://www.rockument.com/kelley.html