KisMAC is a free stumbler application for MacOS X, that puts your card into the monitor mode. Unlike most other applications for OS X it has the ability to run completely invisible and send no probe requests.
KisMAC supports several third party PCMCIA/PCCards cards with Orinoco and PrismII chipsets, as well as Cisco Aironet cards. Original Airport Cards are supported too. The newer Airport Extreme Cards are only supported in a limited active mode.