AirRadar is a free utility that can help you scan for, track information on, and join wireless networks. Like similar apps, AirRadar provides much more information than you'd otherwise get through the Mac OS Airport status menu.
Whether you're testing your signal strength at home or trying to find free Wi-Fi on the road, AirRadar will list all open and closed networks in range, along with information such as signal strength (current, average, and max), type of encryption, MAC address, and channel. AirRadar can keep scanning at any interval you set, automatically join the best open network (or a network of a specified minimum strength), and notify you of new networks in a variety of ways, including through Growl messages and by speaking network names out loud--perhaps a bit dorky, but useful for the visually impaired.
The basic functionality should be more than enough for most users, but you can also have AirRadar provide more than two dozen pieces of live data for a given network, and even track noise and signal strength in graphs. More Mac freeware should be as useful and well-rounded as AirRadar.