I remember seeing a story on TechTV about someone setting up a wireless network free to anyone in Boulder Colorado with a wireless card. I think it was Boulder. It started off with essentially a high teck pringles can and grew to cover the city.
Beware of the signal boosters that are sold in any local dept store, most don't help much. You may want to look for a repeater, they work better than a booster.
Kage is right, you can build one for cheap and you get some chips in the process.