Kismet

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.

A good wireless tool as long as your card supports rfmon (look for an orinocco
gold).

Get Kismet Here - http://www.kismetwireless.net/download.shtml