Although not open as such, BT in the UK have a system whereby their broadband customers run a second access point that's separate to their internal network. Any other broadband customer who opts in to the sharing scheme can use those access points, as well as BT openzone. The idea is that when you're out and about you get free wi-fi from other BT customers in return for sharing your own. Other people's use of your broadband does not come off of your allowance, but you have limits on how much you can use of theirs (to stop you using the public side of your own connection to get unlimited usage!)
Great in theory. Sucks in practice. You either have to keep searching for access points and go through a browser based logon to access them, or download an app that does it automatically. The app is useless, and fails to prioritise your own internal wi-fi when you're at home, connecting instead to the public facing one.
I've tried it, and wandering around my town I found plenty of access points, but only successfully connected to a small handful - or at least, I connected but failed to get internet access.