Kismet for Mac (Kismac)
Every geek that's been around the block once or twice knows that Kismet is the Swiss Army Knife of wireless network auditing tools - not only does it act as a "stumbler," but it can perform brute-force cracks on WEP and WPA encryptions.
I haven't tried it personally as I don't have an Apple laptop (mine's a Toshiba that dual-boots between XP and Red Hat 9.2), but the reading I've done so far indicates that there is limited driver support for Wireless adapters (hmmm, that's why I don't use Kismet on my laptop, I'm too cheap to go out and buy a Prism branded Wifi adapter), but it fully supports the Airport card, and partially supports the Airport Extreme. An added bonus - the above link is to the version that supposedly is made for the Intel-based Macs.
From the creator's site: MacStumbler is a utility to display information about nearby 802.11b and 802.11g wireless access points. It is mainly designed to be a tool to help find access points while traveling, or to diagnose wireless network problems. Additionally, MacStumbler can be used for "wardriving", which involves co-ordinating with a GPS unit while traveling around to help produce a map of all access points in a given area.
Nmap is the most venerable of command-line network scannng tools. Ports are available for Windows, OS X, Linux, and likely for Atari, Comodore, and Amiga systems if you look hard enough. This tool can map networks, and map the open ports on the systems that comprise that network. As if that's not enough, it can enumerate the services running on those ports (banner grabbing), and identify the remote OS through a techniue called "fingerprinting."
Not for the faint of heart (or the feeble-minded), nmap has more options that about any command-line app that I've run across.