One of free Mac WiFi scanner app that released under BSD license allow you to scan availability WiFi or Wireless network around you
iStumbler is one of WiFi scanner app for Mac OS X that able to detect and scan WiFi or wireless network through Airport or bluetooth enabled on Macintosh computer. iStumbler can be used to scan WiFi signal strength, iStumbler is also supports Bluetooth wireless device scanning and Bonjour network service scanning (recent version supported)
When you are in a place like a cafe, mall, coffee shop, or a city park, you can easily to scan for available WiFi network around you, if there are wifi signal detected around you, then iStumbler program will provide you various information about the wifi or wireless network that has detected, such as information network name, signal strength (equipped with a bar level), noise, frequency, MAC address, channel, and protocol in use.
iStumbler – Mac WiFi Scanner
After you found a WiFi or wireless signal, you can directly join the network that you select to be connected to the Internet and browsing the web. You can also let iStumbler program to choose one of the best network to connect for automatically. iStumbler program is under open source BSD license, allowing you see the sourcecode of iStumbler program for just modifications and submit patches to the developer of iStumbler program.
Using iStumbler program is not as hard as you think, you will be only faced with a simple user interface that provide the scanner device selection menu (AirPort, Bluetooth, Bonjour) that you want, and a few settings you can find on setting preferences.