Not the most easily reachable network, but the furthest flung: Results in Venezuela have managed a working Wi-Fi connection over 382 km/238 miles using two points in the Andes. They were using Intel equipment, and achieved 3 Mbps in each direction. Last year, at least one of the same researchers created a 279 km/167 mile link with some funky looking gear. Read this interview with that researcher.
Lincoln Center in Manhattan coins clever marketing name for its Wi-Fi: Linger Longer: Kind of a neat place to noodle around. There's a contest with good prizes that requires you to log on to enter.