http://zeroconf.org/ http://www.apple.com/macosx/features/bonjour/
Zeroconf is a coming together of self-assignment of a (locally) unique IP address, multicast-DNS, service discovery and more. The whole is much more then the sum of its parts.
Zeroconf means that I can walk in to class and sit down and every iBook or PowerBook with WiFi will connect to mine and form an ad hoc network. A zeroconf-aware browser can then ask ''who has a HTTP server?" and get a reply of every zeroconf aware Web server. Likewise for messengers and the like.
AppleComputer brands this technology as Bonjour (formerly Rendezvous before a trademark dispute).