- NYC Mesh - US - https://www.nycmesh.net/

- Freifunk - Germany - https://freifunk.net/

- Guifi - Spain - https://guifi.net/en

Guifi is probably the largest of those, with 36.924 online nodes, most in Spain but some in other parts of the world. If my memory serves me correctly, there is peering working/in progress between Freifunk and Guifi as well.

One example is Freifunk in Germany. There is also a large network in Spain, but I forget what it is called.

There is also this: https://news.ycombinator.com/item?id=25412367 which was recently discussed on HN.