When it comes to businesses promoting their location as having free WiFi access, I think primarily of bookstores, coffeehouses, and hotels.
However, WiFi access is something that any small business can promote. In today's New York Times, there's an article called "In-Store Wi-Fi Is Free, but Not Commercial-Free". It features a company called AnchorFree, which "has introduced a service that lets merchants of any size — from a large bookstore chain to a mom-and-pop restaurant — offer free advertising-supported Wi-Fi to customers on the store premises. People who are shopping or eating in an AnchorFree location will see banner ads on their screens or short video spots or both before their browsing session."
The article also mentions JiWire as a company that "has a similar system, though a different technological approach."
If you have a client whose customer base includes the techno-literate, this is a service that they might want to consider.