AirPcap (www.airpcap.eu)

The AirPcap adapter, together with the Wireshark Network analyser, gives you a detailed view of the 802.11 traffic, including control frames (ACK, RTS, CTS), management frames (Beacon, Probe requests and responses, Association/Disassociation, Authentication/Deauthentication) and data frames. The captured frames include the 802.11 Frame Check Sequence, and it's possible to capture frames with an invalid FCS to spot remote access points with a weak signal.

The AirPcap adapter captures per-packet power and rate information. This is useful for mapping your network, detecting weak signal areas, and measuring the transmission efficiency of the stations.

Thanks to Phil Wiffen - who worked with us at crownhill for many years - there are some excellent articles relaetd to the use of AirPcap and the investoigation of WEP (in)security. - a primer on why NOT to use WEP.

Phil runs an interesting and informative blog , well worth a read