Online file storage and backup on Amazon's servers, and if you upload MP3s you can play them from any computer—or at least any computer that has Flash Player installed. There's a 5 GB free tier, and you can buy up to 1 TB of storage space.
Where this gets somewhat more interesting is that you can have AmazonMP3 purchases automatically saved to your Cloud Drive, where they won't count against your storage limit and can be played back over the web (or via a new Android app) or re-downloaded anytime. And if you buy an MP3 album from Amazon—even one of their infamous $3.99 daily deals—you get a free 1-year upgrade to the 20 GB Cloud Drive plan.
This is really cool, though I hope Amazon will add HTML5 support at some point, and even more that Apple is planning to announce something similar for iTunes content at WWDC.