KisMAC is a free stumbler application for MacOS X, that puts your card into the monitor mode. Unlike most other applications for OS X we are completely invisible and send no probe requests. KisMAC supports third party PCMCIA cards with Orinoco and PrismII chipsets, as well as Cisco Aironet cards.
Please note: This program is not intended for people who don't have much knowledge about WiFi, but for professional users.