IMO the most widely used multiuser app. Two basic approaches

• Centralized (Napster/AG-style) - central server(s) keeps index of all users' files ("who has what"). Search on central server. File download is peer-to-peer.
• Decentralized (Gnutella-style) - each user is both a client and a server ("servent") connected to neighbours and so on. Searches are passed from neighbour-to-neighbour. When a file found it is directly downloaded from provider similar to the Centralized style.

  View the Gnutella Flash Animation