(via www.ubuntugeek.com)
Kismet is a 802.11b wireless network sniffer. It is capable of sniffing using almost any supported wireless card using the Airo, HostAP, Wlan-NG, and Orinoco (with a kernel patch) drivers.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.