http://www.aircrack-ng.org/

Aircrack-ng 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, as well as the all-new PTW attack, thus making the attack much faster compared to other WEP cracking tools.

In fact, Aircrack-ng is a set of tools for auditing wireless networks.



I have seen people use this program for minor stuff, like going into a wifi cafe, hacking the password and fucking with everybody with their connection wise. But it can also be good if you do not have internet and there are locked wireless connections in you area.