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 802.11a, 802.11b, and 802.11g
traffic.

Kismet identifies networks by passively collecting packets. In addition
to standard networks, it can detect (and given time, decloak) hidden
networks, and infer the presence of nonbeaconing networks via data traffic.

Capture sources that are known to be supported: Atheros, Prism2, WSP100,
Drone, wtapfile, pcapfile.  Kismet also supports radiotap headers and
should work with current FreeBSD systems.

WWW: http://www.kismetwireless.net/