You can message and share files inside your intranet without a server.
UFONet - Denial of Service Toolkit
A P2P filesharing client which connects to the Gnutella network.
Mobile P2P App - Use a P2P App on your Smartphone
Metasearch engine for P2P private torrent trackers.
A P2P Messenger which uses JXTA. It provides secure RSA/AES end-to-end
Chat inside a LAN (subnet).