Scanning for wireless networks also referred to as "stumbling", is basically the detection of Wireless LANs using the 802.11b, 802.11a and 802.11g WLAN standards. This is useful for verifying network configurations, locating poor coverage in a WLAN, finding causes of wireless interference, detecting unauthorized access points etc...
In Mac OSX I find these two tools the most useful:
Kismac, stands out because it's a passive network scanner. Making it possible to detect networks without announcing your presence, as well as finding information on closed(hidden) networks.
iStumbler's strengths are in providing stumbling for not only wifi networks, but Bluetooth and Bonjour services as well. A nice addition is a spectrum dashboard widget, which displays a virtual spectrum analyzer for detecting network radio frequency overlap.
MacStumbler is another tool in this category, which seems a bit outdated now, but there may to be a version 2 on the way.