One of the biggest ISPs in Germany, Telekom, offers such a model ("WLAN TO GO") since more then 10yrs. It is an opt-in agreement that you can use other people's

One of the biggest ISPs in Germany, Telekom, offers such a model ("WLAN TO GO") since more then 10yrs. It is an opt-in agreement that you can use other people's APs if you also open yours. Many other ISPs offer the same service, for instance Unitymedia. Furthermore, there are popular nonprofits with similar aims, for instance freifunk.net (despite this is not an ISP). Something I would find very interesting is opening eduroam.org for other organizations/private people. It is the worldwide dominating "shared WiFi" approach in the scientific domain.