Fon is a network that provides free and $3/day web access through wireless access points set up in homes and businesses. If you provide an open AP you can use any other for free, if you charge for access, you have to pay. The network has grown fairly rapidly. If you would like to join, do so before 8th November, 2006, as that is the last opportunity to purchase a subsidized €5 router. The new price is €29.95, so a decent saving.
A venture such as this could prove extremely useful for mobile VoIP. We’ll just have to wait and see if it interests enough people to make it useful.
For more information check out its Wikipedia entry.