I just recently watched a couple of double features, one being two Roger Corman films, The Young Racers (1963) and The Wild Angels (1966), the other being an excellent pairing of The Trip (1967) with Psych-Out (1968). Of the four, The Trip (also directed by Roger Corman) is by far the best, followed by what is sort of a cheesy precursor to Easy Rider, The Wild Angels, featuring Dennis Hopper and Peter Fonda, next being Psych-Out with Jack Nicholson, lastly, The Young Racers, the worst of the four.
All four have a kitschy, retro appeal for me, but The Trip stood out for, among other reasons, having an interesting soundtrack. Most of it is, as you would imagine, heavily psychedelic. But that's good, because I enjoy the genre! The soundtrack is by The Electric Flag, a short-lived group formed by Mike Bloomfield in 1967. I'm rather partial to "Peter's Trip," sort of a regal, harpsichord-driven blues jam, at times sounding like an outtake from Pet Sounds. It's one of those songs I feel like I've heard before, although I don't think I ever have.