Aircrack-ng

Aircrack-ng is a tool pack to monitor and analyse wireless networks around you and put them to the test. Check how safe your wireless password is or unlock your neighbour's wireless network. It can guess WEP (Wired Equivalent Privacy) and WPA (Wi-Fi Protected Access) passwords.

Once you run the program, you'll be able to capture data packs of a certain network thanks to one of the tools in the pack: Airodump, an sniffer that willl collect data packages so you can exam them later. Once you'll have captured enough packages, you only have to load them on Aircrack and it will decode them and will tell you the password of that wireless network so you can access it.