NetSpot is a free tool for discovering local wireless devices and creating colour-coded heatmaps.
Launch the program and it immediately detects all your local wifi devices, displaying their details in a table: SSID, BSSOD, Signal (current/ min/ max/ average), band, level, width, vendor, security, mode, and when it was last seen.
Clicking any column header sorts by that field. For example, clicking "Channel" groups devices by their channel, helping you spot competition and conflicts.
This isn't just about static figures. NetSpot displays tiny signal graphs within the table, and double-clicking any device shows graphs and tables of its signal over time, along with other interesting details (when and if the channel and security were last changed).
There's also a separate Survey mode which enables collecting and displaying wifi data on custom maps.