Somebody reports:
"It is a stumbler program for Mac OS X. It is a beta but I have had no problems with it. It produces a log that came be saved to disk. To my knowledge it is the best stumbler for WiFi for Mac OS X out on the web (and it's free.)"

Edited by Synthogen:
"There are a few minor bugs with it. One such bug is the inability to add spaces in the comments field; you must enter underscores instead of spaces. Second, I don't believe there really is any other Mac OS X stumbler, everything else is for UNIX."
But see below for two more.

Edited by (tiny_key/bb):
There is now KisMAC an entirely passive wireless scanner for Mac OS X, it's still alpha but has some great features, including WEP cracking, passive scanning, and an option to allow you to close the lid on you mac laptop and still stumble (something Mac's don't normally do).

And also iStumbler which support AirPort and Bluetooth sniffing as well as providing a graphical display of signal and noise over time.