Amazon recently announced their Cloud Drive and Cloud Player. This service provides 5 gigabytes of free storage to all users with an Amazon.com account. If you purchase one MP3 album per year, you get 20gigs of space (additional space is priced at $1 per gig per year, up to 1 terabyte). Amazon has provided an application that, once downloaded, searches your Mac or Windows PC for music files. You then upload playlists or your entire library to your Amazon Cloud Drive. These files are available to play from any internet enabled computer. Amazon has already provided an Android app, and one can assume that they are working on an app for iOS.
This is not just a “player.” This can work as on online hard drive. If/when your hard drive dies, you can redownload all the music that you have uploaded to your Cloud Drive.
This service seems more useful as a storage solution, rather than music (and video) player at this point. Once mobile apps are more readily available this will become a much more viable solution for people that have large music libraries and relatively small storage on their phone/iPod/iPad.