Distributed Internet backup

Consider a distributed computer program like the following.
1. You download the program an install it.
2. During install it asks you how much space you want to share for others to use and which folders you want to backup.
3. It then starts sending your data (encrypted) to other users on the network (the client would probably have to send the data to multiple places and keep track of where it is).
4. It also starts accepting backup data from other users.
If you need more space than you can provide then you can pay extra. If you provide more space than you need then you can get paid.

Then your backup data is distributed across the internet. I think it would be useful to many people!

Would this work well? Does it already exist?