While not a new concept, a new decentralized service is beta testing a peer-to-peer network that would "rent" unused capacity from your computer's hard drive as part of a cloud service to store files from other users.
symform - News, Features, and Slideshows
I’m still in the audience at the DEMOFall conference, and still taking in demonstrations of new products and services. One of the cooler ideas this morning is Symform, a small-business remote-backup service. Technically, it’s utterly unlike services such as Mozy and Carbonite: Those services store everything in massive server farms, but Symform is farm-free–it uses peer-to-peer technology to store backups on the PCs of other Symform users. If you wanna back up 10GB of data, for instance, you agree to devote 10GB of disk space to other folks’ backups–and to leave your computer on 80 percent of the time.