Kismet

Kismet is a console (ncurses) based 802.11 layer-2 wireless network detector, sniffer, and intrusion detection system. It identifies networks by passively sniffing (as opposed to more active tools such as NetStumbler), and can even decloak hidden (non-beaconing) networks if they are in use. It can automatically detect network IP blocks by sniffing TCP, UDP, ARP, and DHCP packets, log traffic in Wireshark/tcpdump compatible format, and even plot detected networks and estimated ranges on downloaded maps. As you might expect, this tool is commonly used for wardriving. Oh, and also warwalking, warflying, and warskating, etc.