Not just Europe– I had a test unit from way back for when they were entering the US, though they never got really big (I do see them occasionally). In Japan I see them everywhere.
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."
Not exactly because you don't have the ratio idea where you have to share XXX MB with someone before having YYY MB for yourself.
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.