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.