AirStatz is an app that can give you some information about wireless networks. The app uses the built-in Airport card on your Mac to scan for wireless networks. It then displays them and shows some basic information about them like SSID, strength, channels, etc.
The main window shows a big circle which changes colors depending on the signal strength and the interference that you get while connecting to a wireless access point. If the signal to interference ratio is good, most of the circle will be green and the rest red. If the link is dead, you will only see a red circle. Unfortunately, when I tested this app, all I saw was a red circle. I was connected to my wireless router and browsing the Internet, but the app still showed a big red circle.
The information that you are shown isn't really presented in a way that is easy to read. The names of the networks appear with a string of numbers before them, and the rest of the information looked messy, and you have to constantly click on different categories to access the information.