I became aware of Jesus Freaks around the time Andrew Lloyd Webber unveiled Jesus Christ Superstar. It all seemed so very Vatican II: it was suddenly okay for Jesus to rock.
Jesus Freaks were hippies who turned away from the sex, drugs and rock'n'roll of the 60s and 70s and instead embraced Christianity--or at least their interpretation of it.
Some Jesus Freaks continued to follow their rock muses, forming bands and making records. In many ways, these people created the foundations for today's Christian rock scene.
There are people who collect this stuff, too. Boing Boing has an interesting article on the entire phenomenon. Have a listen to this sample from Azitis.