I got my FON router delivered to my house about 2-3 months ago, but I didn't realize what it was until I was cleaning up my office to leave for Buenos Aires. Glad to have a project, I set it up on my home network while I was waiting for the taxi to the airport. I've only really started playing with it since we got back.
FON is an international effort, originally based in Spain, to share extra network capacity with roaming wireless users. You set up a router in your home (or wherever), and any other FON user can connect to the Internet through your network. Conversely, you can connect on any other FON network. There is a lot of grass-rootsiness to FON, but it's a commercial project, which turns some people off.
I originally heard of FON at Wikimania 2007, where FON provided wireless routers for each attendee's room. It was a nice service, although the doubled-up ESSIDs were kind of confusing at the time. They make a lot more sense for home use.
I was impressed by how many FON spots there are on the Montreal/Plateau (see http://maps.fon.com/ for details); about one ever 2-6 blocks in any direction. I think there's an opportunity here to see a higher concentration, though. I'd love to see coöperation between FON and, say, Île Sans Fil, possibly involving the Ville de Montréal. Apparently there are already FON cities in different parts of Europe, so it's not impossible.
I think FON is an interested development for WiFi -- a hybrid between community-based initiatives and a commercial venture. I'm not sure if it will be successful, but I think it's a step in the right direction.