"Kismet is a 802.11b wireless network sniffer. It is capable of sniffing using almost any wireless card supported in Linux, including Prism2 based cards supported by the Wlan-NG project (Linksys, Dlink, Rangelan, etc), cards which support standard packet capture via libpcap (Cisco), and limited support for cards without RF Monitor support.

Features:

Multiple packet capture sources
Runtime network sorting by AP MAC address (bssid)
IP block detection via ARP and DHCP packet dissection
Cisco product detection via CDP
Ethereal and tcpdump compatable file logging
Airsnort-compatable "interesting" (cryptographically weak) logging
Secure SUID behavior
Cloaked network detection
Grouping and custom naming of SSIDs
Multiple clients viewing a single capture stream
Graphical mapping of data
Cross-platform support (handheld linux and BSD)
WEP decoding"

www.kismetwireless.net