In a spot where I lived where there were 20+ networks visible, there was one very strong open network (they were obviously using a substantial antenna or two) that was around for more than two years. It was quite popular - you could routinely see a dozen or so clients connected in the evening. Connecting to that network would often yield a OS finger print scan + occasional attempts to exploit vulnerable services. Traffic going over router also very obviously was having its adsense traffic replaced. God only knows what else they were doing. Since it's on their network, it's very likely a lot of that wouldn't even be illegal.

I certainly wouldn't go around encouraging people to connect to random wifi networks. It's a different world out there than it was 5 or 10 years ago.