If you run [Linux] then Kismet is an excellent tool for WarDriving or site surveys. (The BSD's, MacOSX and Win32 (Cygwin) are also mentioned, but with varying degrees of support.)

From the (http://www.kismetwireless.net/ Kismet home page:)
Kismet is an 802.11 layer2 wireless network detector, sniffer, and intrusion detection system. Kismet will work with any wireless card which supports raw monitoring (rfmon) mode, and can sniff 802.11b, 802.11a, and 802.11g traffic.

Kismet identifies networks by '''passively''' collecting packets and detecting standard named networks, detecting (and given time, decloaking) hidden networks, and infering the presence of nonbeaconing networks via data traffic. 

Common applications Kismet is useful for (as extracted from the (http://www.kismetwireless.net/documentation.shtml documentation))...
* '''Wardriving:'''  Mobile detection of wireless networks, logging and mapping       of network location, WEP, etc.
* '''Site survey:'''  Monitoring and graphing signal strength and location.
* '''Distributed IDS:'''  Multiple Remote Drone sniffers distributed throughout       an installation monitored by a single server, possibly combined with a       layer3 IDS like Snort.
* '''Rogue AP Detection:'''  Stationary or mobile sniffers to enforce site policy against rogue access points.

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Other related software at WarDriving