KisMAC is an opensource and free stumbler/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 Prism2 is supported as well, and USB Ralink support is in development. All of the internal AirPort hardware is supported as well.
Please note: This program is not intended for novices, but for professional users. Take this as a warning

System Requirements

• Mac OS 10.4
• A Mac with a supported PCMCIA, USB or internal AirPort

Features

• Reveals hidden/cloaked/closed SSIDs
• Shows logged in Clients (with MAC Addresses, IP addresses and signal strengths)
• Mapping and GPS support
• Can draw area maps of network coverage
• PCAP import and export
• support for 802.11b,g,n
• different attacks against encrypted networks
• deauthentication attacks
• AppleScript-able
• Kismet drone support (capture from a Kismet drone)

Supported hardware chipsets

• Apple Airport, AirPort Extreme, AirPort Extreme Atheros
• Lucent Hermes I & II (Orinoco 2 and 11MBit)
• Intersil Prism 2, 2.5, 3 in PCMCIA and USB devices
• Cisco Aironet (with older 4.xx firmware)
• Intersil PrismGT
• Atheros PCMCIA
• Ralink USB (usb-drivers branch only)

Crypto support

• bruteforce attacks against LEAP, WPA and WEP
• weak scheduling attack against WEP
• Newsham 21-bit attack against WEP