From Lifehacker (old version): "Leopard's built-in Airport utility doesn't do much except show you what Wi-Fi networks are nearby, and which require a password—but free utility AirRadar does much more. AirRadar detects available Wi-Fi networks (in my case, more networks showed up in AirRadar's list than in Leopard's default menubar drop-down), and displays more information like signal strength, last seen timestamp, channel, signal average, max, and min. Disconnect from networks without turning off your Airport card, get Growl notifications, and set the scan interval with AirRadar too. You can even save networks as favorites, instead of just hopping onto any old open network by default. If you've got a spotty signal you'll appreciate AirRadar's signal to noise line graphs, too. AirRadar is a free download for Mac only."