Since I'm spamming with this same idea right now - the description is similar to Freenet (the old one, the Hyphanet), but you'd need some kind of ability to choose parts of which collections of data get stored in your contributed storage, while with Freenet it's all the network (unless you form a separated F2F net, there is such an option, but no way to be sure that all peers, ahem, store only IA data and not their own porn collections, for example, taking precious storage). I've described one idea in my previous comment, but it's purely an idea, I'm nowhere close to having the knowledge to make such.