802.11 -- "WiFi" -- is cool. We love WiFi. W1r3l3ss r0xx0rs. Etc. But it suffers from an occasionally significant limitation: you have to be near a WiFi router to take advantage of it. If there isn't an open WiFi network nearby, tough luck. But... what if your wireless device could discover other wireless devices in the vicinity, which in turn know about other devices, and so forth, until you get to one within range of a WiFi router? You could then pass packets along, using this ad hoc network without actually being in range of a "real" network.
Such a day may soon be upon us. The Networks and Telecommunications Research Group at Trinity College in Dublin, Ireland, is building out an experimental system for combining a variety of wireless protocols into a city-wide voice and data communications grid -- the Dublin Ad-hoc Wireless Network, or DAWN. With this network, mobile devices from phones to PDAs to laptops could pass packets along a constantly-reconfigured route from user to user, including moving from the DAWN system to the fixed voice and data networks. It's all very experimental and tentative at the moment, but it certainly has some potential.
The notion of ad hoc networks isn't just a laboratory hack. The Cybiko, a $99 handheld game/MP3/PDA/chat system aimed at the teen mallrat market, uses the ad hoc network method to let up to 100 devices trade messages, music, and games at any given time, as long as the various members are within 300' of another network member. The current model doesn't connect to WiFi, requiring a serial connection to a PC to send anything to the Internet at large, but I wouldn't be surprised it that was in the next iteration of the device.
There are already commercial versions available now:
Developed for the Army, as I recall.
And there are Wifi/Linux solutions too. See the MeshAP Linux distro from http://www.locustworld.com. It uses an Ad-Hoc routing protocol to allow the Wifi network to grow more organicly. It's how we brought WiFi to Brighton beach: http://www.piertopier.net