I think the Beatles were definitely the forerunners of the psychedelic sound. But, although somewhat psychedelic, it was still focused on the mainstream audience. Pink Floyd took it one step futher, at least in their early days, creating long, drawn out trip music. So, while the Beatles music was heavily influenced by their psychedelic experiences, Pink Floyd songs like Astronomy Domine, Intersellar Overdrive, etc. were meant to be psychedelic experiences in themselves.

Paralleling the English psychedelic sound was the San Francisco psychedelic sound. Similar but both unique. Jefferson Airplane and the Grateful Dead are the best known but there are a lot of others. Big Brother and The Holding Company and Quicksilver Messenger Service are great too.

Another great psychedelic tune is Paul Butterfield Blues Band's East West. Check it out.

Hare's a link to a live recording of the Grateful Dead playing a free concert in San Francisco's Golden Gate Park back in 1967. http://www.archive.org/details/gd67-...108.sbeok.shnf
This was called the "Human Be-In" and was basically a giant acid party. Signifigant enough to warrant a wikipedia entry http://en.wikipedia.org/wiki/Human_Be-In