Fon asks its customers to install Fon-powered Wi-Fi access points on their broadband connections, and it makes them all into a global shared network. Whisher, by contrast, collects the access keys to the existing routers that its subscribers already use. In other words, to put your broadband connection on the Whisher network, you give the service your Wi-Fi access point's ID and security code (WEP or WPA key), and then Whisher holds that information in proxy for its other subscribers.
Subscribers use the Whisher software to connect to access points on the Whisher network. The actual passwords that Whisher transmits are not revealed to subscribers.
Clearly, it's a much simpler way to build a co-op network of access points, since it doesn't require dedicated hardware.
It is, however, a scary proposition from a security perspective. With Whisher, anyone who gains access to a Wi-Fi access point can put it on the network. Whisher founder Ferran Moreno said, "As long as you can connect, we assume you are the owner." But that's not the case: Once you give your network's Wi-Fi key to friends or employees, any of them could put your Wi-Fi on the Whisher network, and you might never know it--until you find strangers on your LAN and no bandwidth left for the people who own it.
Whisher has a "private mode" that you can use to turn off access to a router on the Whisher network, but if you haven't installed it, I don't see how you'd know about it.
Whisher is a free. The company makes money by monetizing location-specific Web content, such as a portal that has links relevant to the location from which you are connecting, and via buddy-finder, instant-messaging, and file-sharing utilities.
Whisher is a really clever way to build a global network of free Wi-Fi access points that share its users' broadband connections. But I don't think I am being too paranoid about its security issues.