By Russell Harris.
OS X Lion released a handy new Wi-Fi Diagnostics app hidden inside /System/Library/Core Services. However, this has been improved in Mountain Lion with an additional Wi-Fi network scanner and a quick way of accessing it.
Now in 10.8, you can easily open this hidden Diagnostics Tool by holding down the Option/Alt key whilst clicking on the Wi-Fi menu bar icon. The addition of the Option/Alt key opens up some extra info to the Wi-Fi menu bar icon.

Choose “Open Wi-Fi Diagnostics…” at the bottom to quickly load the diagnostic tool.
You’ll now be faced with the Welcome dialog window of Wi-Fi Diagnostics allowing you to run Diagnostic Reports, in-depth debugging Logs and even capture network traffic.

New to 10.8 is a Wi-Fi network scanner, accessible by selecting ‘Network Utilities’ from the File menu or by pressing the ‘Command+N’ keys.

This new Network Utilities screen includes a Wi-Fi stumbler tool, a signal and bandwidth meter, bonjour scanner, and a variety of other useful network tools like ping and traceroute.
Pretty much everything you need to monitor and troubleshoot your Wireless connection is provided here without having to download and purchase additional tools.
The Wi-Fi scan option is handy to see what Wireless networks are in range, with what channel and frequency band they are using, so you can optimise your own Wireless network to avoid conflicts with other networks in range.
This feature has been tested using Mac OS X v10.8.0 which was the latest Mac OS release at the time of writing and also using the latest Internal and external Apple keyboards.