To coincide with the BarbieGirls.com website, Mattel decided to release MP3 players themed around the site's avatars. They were simple MP3 players with only a few megabytes of storage (a special one gigabyte version was available), and they had no screens, but the gimmick was that they were customisable. You could switch around the fashions on the player, and purchase extra fashion packs to get more outfits for it.
Each MP3 player came with a pair of headphones, a docking station, extra fashions, and a disc. I'm not sure if the disc contained software to add songs to the device, or if it was used to connect the MP3 player to BarbieGirls.com too. Like most toys based on online games, these MP3 players gave you perks in BarbieGirls. I believe they came with VIP memberships and the ability to unlock "Secret B Chat", which meant that you could chat with a friend privately, however I think they also had to own an MP3 player for it to work. The extra fashion packs also contained unlockables like a free in-game pet.
I've never actually owned one of these, except I do own a mini doll sized one. This miniature one doesn't work, obviously, but it came with a 2008 Barbie My House playset. It's quite cute and it's the only piece of physical BarbieGirls merchandise that I own.
Nowadays you can find these used on eBay and probably in a flea market or charity shop somewhere. I'd like to find one someday, even if it is a very useless item nowadays.