I am trying to find a quick/easy/idiot-proof way to set up a true P2P network such that all traffic is restricted to a local subnet. This should preferrably be done from the client-side with a config file.

I am looking for a solution that is true P2P (that is, doesn't require a centralized host that everyone on the network has to connect to), and that is not much more difficult for users to setup than something like "Download this package, run the setup, select which files you want to share." This solution should preferrably be OS-independent. (or at least work on windows and mac). Has anyone encountered something like this before? Wanting to be able to share, and easily search for files on a LAN? (Not simply windows filesharing) Perhaps a gnutella client packaged with a config file?