My tutorial for Ubuntu Gutsy which uses aircrack-ng will give you full instructions for quickly obtaining a wireless network key, but I highly discourage you to abuse this knowledge.
- You should only use it if for some reason you couldn't connect to your own DSL connection and quickly need to lookup some help on the net.
- Or you're in nowhere, far away from your own network connection and quickly need some information or need to send an important email.
- There will be no harm, if you use someone's network connection for a few minutes, without using a high amount of bandwidth.
- You shouldn't start to use P2P downloading for example.
Of course, you could use it also to check your own wireless connection setup to see how quickly someone could crack yours.