I must chime in here... I agree that the grunge movement in the early 90's made drop D tuning popular. But... Eddie Van Halen is the guy who made it cool originally, just not many picked it up back in 1978...
Jerry Cantrell actually took lessons from EVH, and picked up on the drop D tuning, and figured out how to use it properly. Kim Thayill has done it for years. Godsmack was formed as basically an offshoot of the AIC tone.
So, EVH was the guy to use it first, then the grunge movement picked up on the heaviness of it and made it popular...
I must also say that the earliest guy I can think of to use low tuning was Albert King. His V tuned to C was amazing, given the time it was done. Many guys were influenced by his tone through the years, so through his influence, many used it. However, in a modern sense, Black Sabbath are the first to use it in a hard rock venue, because of the Iommi accident of lopping off the tips of his fingers, making it difficult to bend strings. So the lower tuning was to facilitate bends, but added a real heavey tone to their music, giving them an original sound from other bands through the 70's.
So, I would say they made low tuning popular....
ax. :twisted: