Air stumbler is fun. I've also used macstumbler which is a good app and also free, but air stumbler has a brushed metal look and the ability to join networks as opposed to just detecting them.
Try driving around neighborhoods with airstumbler open to discover wireless networks, quite fun.
I feel like it needs a little more polish to be perfect -- I think the signal and quality meters are reversed in the drop drawer, and I had trouble getting it to consistently play a noise when detecting a new signal. However it's a must have -- I think -- for anyone with a wireless connection or traveler w/ wireless. Certainly a better meter of signal/quality than os X's built-in tools, and for that it's useful for obtaining the best signal on your own network.