Active ingrdient: camphorated brimstone! You can still buy 666 here and there, but some folks say it hasn't been the same since they took out the quinine.
How did it get the name? According to this piece - which involves a long ten-year old account of ribbing the brand's owners - it was one of those Rolling-Rock-32 things:
"The first order was written on an order tab with the number 666. The product worked. People asked for the product by its order number and it started to be called 'that 666 product'. The name 666 stuck."
I don't believe it for a second, but it's a nice story.