Two different, broad-based approaches to community-based WiFi sharing:

Whisher
You don’t need to leave your WiFi open in order to share it with others. Simply enable encryption on your access point or router (WEP/WPA) and Whisher will do the rest. From this time onwards only you and other Whisher members will have access to the Internet through your WiFi.
Whisher takes a software-based approach to community-based sharing, and supports any router, Macs, PCs and Linux. The software on Macintosh is elegant and usable, although I found it a trifle unstable, slow and processor intensive when setting it up. So far I haven’t succeeded in registering my home access point as my own…

FON
FON provides inexpensive routers (“La Fonera”) that can be easily connected to your home broadband connection. In return for sharing your own connection, you can either make money on visitors (useful for cafes and so on), or get free access at other FON access points.
From FON:
  • Easy to install 54 Mbps wireless router
  • Get free WiFi access wherever you find FON
  • Decide how much bandwidth you share
  • Share safely and keep your own private connection
  • Enjoy our community features and meet other FONeros
La Fonera works well. It’s simple to setup and reliable, supporting both an internal, private network and an outside network accessible to visitors. The only downside? At least in the broader San Francisco area, I’ve never found another Fon access point to use, although the Fon maps show some.