Napster. Going from listening to country/pop music because that's really all you could find on the radio to + mail order CDs you had no idea what they were before you bought them to... being able to sample the entire world's musical various cultures and subcultures as soon as you heard of it...you could tell not only that things like dubstep & machinae supremacy were in the pipe...but that everything outside of music was going to be different after that.
@jeffcliff oh heck yes! Napster completely changed my relationship with music! I tried something new almost every day for a long time... and it was definitely the first time I felt able to see trends forming and watch those musical ideas spread.