Minidisc. My then-boyfriend bought me a recorder for Xmas 2002 and I was absolutely in love with it. I still use minidisc but I think for most people it was a flash in the pan - it didn't have the staying power that I thought it would.
Also ephedra - right before it was banned, all the women in my office went through the Atkins -> Xenadrine -> Pilates fads. Holy crap did that suck. Atkins made me stink (ketosis) and xenadrine make me shaky, dizzy, and irate. I was too poor for pilates haha. The entire office was miserable!