Here's an old classic from Cream's debut album. "I Feel Free" was the first single off the album, their second single ever as a band. The overdriven tone of Eric Clapton's guitar during the solo was incredibly heavy for 1966. To compare, The Beatles' song "Revolution", which featured heavy overdrive, wasn't released until 1968. Blue Cheer's cover of "Summertime Blues," considered to be the very first heavy metal song, wasn't released until 1968. And Led Zeppelin didn't get their start until. . . you guessed it - 1968. Eric Clapton's wailing guitar solos were several years ahead of their time, one of the many reasons he was considered a pioneer during the 60's.
You wanna know the only other person who used that kind of tone in 1966? Jimi Hendrix.