iPod. It was the first commercially available MP3 player that sported more than 512mb of storage. First model was 5GB. Second was 10GB.
I got in on the second model, as a Windows PC user. I had to buy a FireWire expansion card just to use it.
Literally nothing else was like it, and at the time, you could leave it on the seat of your car while you went shopping because that far back, nobody knew what the fuck it was and so would leave it alone.
They didn't create the first MP3 player, but they created the first massively commercially successful one.
Through this, they also pioneered the first digital storefront for music which in itself was a fucking feat considering there is already a music company named Apple. They threaded the fucking needle with that one. They had trademark disputes with Apple Corps (holding company for music by The Beatles) going back to the 1970's but put that all to bed with the release of the iTunes store.