What it is: Kismet is an 802.11 Layer 2 wireless network detector, sniffer and intrusion detection system. It works with any wireless card that supports raw monitoring mode.
Key features: Kismet is able to identify networks by passively collecting packets and detecting named, hidden and non-beaconing networks via data traffic. The software can provide graphical mapping of networks, perform hidden network SSID de-cloaking and output data via XML.
Where to get it: Kismet is available for download here via GNU GPL under the GNU Privacy Guard Key.