iStumbler 98: "iStumbler is the leading wireless discovery tool for Mac OS X, providing plugins for finding AirPort networks, Bluetooth devices and Bonjour services with your Mac.
Release 98 includes a re-written AirPort plugin which now correctly reports signal for all current AirPort chip-sets, and provides more information about all networks. The info window is now resizable and the AirPort Inspector shows dBm values when available."