Game Key Revealer makes it possible to extract keys/serial numbers for games that are installed on a computer. It works for over 500 of the most popular games you can install on your computer. Just run the program, press the Find Games button and it'll scan the computer and show all of the keys/serial numbers it finds for games on the computer.

It goes without saying that this is for personal use only on games that you have legally bought. That said, I could imagine some other uses too :)