I recommend using a Live DVD image on a laptop with WiFi.
Cracking WEP (Lifehacker)
Cracking WPA (Lifehacker)
This requires the router to have WPS’ PIN enabled, which many tech-savvy users are disabling. I can promise there are still many routers with this enabled. On my block, there are 12.
NOTE: Don’t do this to networks you are not authorized to access and/or test. I do not take any responsibility for any legal ramifications.