I wouldn't describe client-server as p2p.

Opera created a peer-to-peer browser not long before going under, Unite IIRC. The browser was both a client and server, you had a virtual fridge to put messages and photos on, could open folders on your computer to any subset of your chosen peers, etc..

I was really excited about the prospects it seemed to offer; I blogged about it at the time, http://alicious.com/opera-about-to-change-the-world/.