I know it's here somewhere....

Vembu Systems recently announced a new version of their StoreGrid software which will backup your files to other computers on the network using their spare disk capacity.
I had kind of abandoned this idea several years ago as, in my opinion at the time, the major use of it may have been questioned as unethical.

It is probably an idea whose time has come. StoreGrid is fairly basic and only allows discovery by multicast, followed by scheduling of backups to the peer machine using encryption and compression.
Given the amount of excess disk capacity out there, it's a great way to empower users in a small organisation to back up important files. How many times have you heard the excuse that some file or other was inadvertently deleted, corrupted or the laptop crashed?
What StoreGrid doesn't do, that I had envisioned back in '98, is the following:
- Use software RAID to spread the files over multiple machines redundantly
- Produce a very small 'backup key file' (password protected) with the locations and decryption key necessary to put the file(s) back together again (this would mean you could store your files on the net, but they would only be accessible in their entirety to the person with the key file. It's a bit 'James Bond', imagine you want to protect some files you have created/found on a public computer (internet cafe or corporate server); just pass them to the software, it will encrypt and split the files into small pieces and save them to multiple peers. It then provides a key file which you save to your USB flash disk and you can delete the backed-up files on the public computer. Saving may also be faster depending on the internet connection speed (as you are saving in parallel)
- Have a central server that tracks the online status of available peers and their reputation (for reliability and availability).

I was in Sweden at the time and had recently had my laptop stolen, with several highly sensitive product roadmap documents on it. It was a solution to that problem, but it also provides a means for illegal material to be completely hidden from the eyes of the authorities. Nowadays I am more cynical about the need for the authorities to see everything that exists in electronic form.

There are similar systems to Vembu out there, but the P2P aspect of backup is the interesting part. If Vembu add the security angle they might get more attention.