aircrack is an 802.11 WEP and WPA-PSK keys cracking program that can recover 
keys once enough data packets have been captured.  It implements the standard
FMS attack along with some optimizations like KoreK attacks, thus making the 
attack much faster compared to other WEP cracking tools. In fact, aircrack is
a set of tools for auditing wireless networks.

WARNING: Only to be used for surveillance of your own network.  Use with care.