USB drives. I used to get together with a group of friends every Friday night and one night one of guys shows up with a thumb drive. It was an early one, something like 64MB capacity and expensive. Holy crap we laughed at him. "People will just burn CDs to share files" we said. "People will just use email" we said. I was firmly in the "this is stupid and will never catch on" camp. Yeah.
In 2008, I remember telling everyone that there was no way in hell that the "average American voter" would vote for someone named "Barack Obama" -- I really whiffed on that one.