Kismet
Kismet is a pretty decent tool to have in your wireless arsenal.
It's mostly used for identifying wireless networks in the area and gaining as much information about them as possible even if the information is not supposed to be available.
This helps drastically when you start to actually attack the network/device with aircrack or something similar, you already have all the information you need.
Feature set:
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)