A new Wi-Fi scanner tool is in Mountain Lion’s refreshed Wi-Fi Diagnostics Utility, allowing users to easily discover Wi-Fi networks within range and view related data not available from Apple’s Wi-Fi menu bar item. Comparable to third-party Wi-Fi stumbler tool iStumbler, the scan tool provides data for BSSID, band, protocol, channel, signal strength, security, and more. It also has Active and Passive scan modes.
From the window, you can see what networks are in your area and their strength. Some maneuvering of base stations can increase coverage to the outermost areas of your house, and it is much, much easier to see slight differences in numerical signal vs. noise strength over those four waves Apple uses in its Wi-Fi menu.
Additionally, by looking at what Channel (1-13) your base station is on, compared to your neighbors (or other base stations in your house), you can often find the least used channel in your area to improve reception.