Well, it's rather like karma: "As a member of the Fon community, you agree to share a little bit of your WiFi at home, and get free roaming at Fon Spots worldwide in return."
In France Free is a well-known ISP offering this kind of feature with its ADSL box. You open your WI-FI to others and then you have access to every other WIFI shared by users from this ISP.