WebWorkerDaily.com provides a great list of franchised businesses in the USA that provide reliable WiFi at all of their locations. According to the author, Judi Sohn, "Admittedly, these are franchises with locations in the Northeast United States." She goes on to describe the ambiance of the Panera bakery on Nassau Street in Princeton, so she must live and work around here.

Other businesses she points to as having reliable WiFi in some or all locations are Starbucks (obviously), Borders, Barnes & Noble, McDonalds, and Cosi. Note that she is not talking purely about places with free WiFi. In fact, most of these establishments charge for WiFi access in some fashion.

There's good reader comment here about other businesses where free agents and other business travelers can get online around the USA and the world.

Update: Andy Abramson expands on the information provided by Judi Sohn by telling which providers have good throughput in addition to basic WiFi access. Throughput is the name of the game. Who cares if you can get on a network if the responsiveness of the Internet is lousy from that network?