Is it your wireless router that you misplace the key for ? if it is you can find it out by connecting to it with a ethernet cable putting the routers ip address into your browser. Logging in with credentials and finding the key.
You could always press reset on the back of the router.
It'll restore it back to factory default.
Alternatively if it's hacking someone elses wireless, well... thats illegal.
I know Microsoft do run Wireless network administrator courses that do include wireless (WEP and WPA) password decryption, by using an airsniffer and Linux, because my company paid for me to do them. Fun week :-). Supprisingly easy to uncover the passwords..
But don't do it.
Get use your own.