Tinkernut demonstrates how to crack a wifi connection, as well as how to better secure your own personal connection. Also included in the video gallery is Tinkernut's previous tutorial on the same subject.
LifeHacker posted a great detailed text guide to accompany the second video below, along with these words of advice:
"Knowledge is power, but power doesn't mean you should be a jerk, or do anything illegal. Knowing how to pick a lock doesn't make you a thief. Consider this post educational, or a proof-of-concept intellectual exercise."