As I noted in the listing of Wi-Fi directories, Wi-Fi-Freespot Directory, www.wififreespot.com, focuses on free hotspots. This is a good starting place if you are looking for no-cost Wi-Fi public access. It is also worth noting that some of the big telecom providers give free Wi-Fi access to users who subscribe to some of their other services. For example, Verizon provides free Wi-Fi hotspots in many downtown New York locations to their DSL subscribers.
Most of the free hotspots fall into one of three categories:
Provided by a business in the hope of drawing traffic
Provided by a library as a public service (see the following discussion)
Provided by a local community (see the following discussion)
It's really worth browsing ...