I'm no marketing guru, but this seems like a no-brainer. If your library has free wi-fi service, advertise it online to help visitors to your area find out about it. MaintainIT has a list of free sites where you can list your library as a free wireless hotspot, and there are many more.
Is your library's wireless service listed here?
On the flip side -- if you're planning a summer vacation in Wisconsin, keep local public libraries in mind for wireless access. Many of them offer it!
Thanks to Cheryl Becker for following the blogs for this tip.