Reaver is a WPA attack tool developed by Tactical Network Solutions that exploits a protocol design flaw in WiFi Protected Setup (WPS). This vulnerability exposes a side-channel attack against Wi-Fi Protected Access (WPA) versions 1 and 2 allowing the extraction of the Pre-Shared Key (PSK) used to secure the network. With a well-chosen PSK, the WPA and WPA2 security protocols are assumed to be secure by a majority of the 802.11 security community.

Download
http://code.google.com/p/reaver-wps/downloads/list

Usage is simple just specify the target BSSID and the monitor mode interface to use:

# reaver -i mon0 -b 00:01:02:03:04:05

Info
http://securityxploit.blogger.de/stories/1970771/