KisMAC is an open-source and free sniffer/scanner application for Mac OS X. It has an advantage over MacStumbler / iStumbler / NetStumbler in that it uses monitor mode and passive scanning.
KisMAC supports several third party PCMCIA cards: Orinoco, PrismII, Cisco Aironet, Atheros and PrismGT. USB devices with Intersil Prism2, Ralink rt2570 and rt73, and Realtek rtl8187 chipsets are in progress towards full support as well. All of the internal AirPort hardware is supported for scanning.