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.
Features
802.11b, 802.11g, 802.11a, 802.11n sniffing
Standard PCAP file logging (WireShark, Tcpdump, etc)
Client/Server modular architecture
Multi-card and channel hopping support
Runtime WEP decoding
Tun/Tap virtual network interface drivers for realtime export of packets
Hidden SSID decloaking
Distributed remote sniffing with Kismet drones
XML logging for integration with other tools
Linux, OSX, Windows, and BSD support (devices and drivers permitting)