Pretty much all commercial software that ships today comes with some sort of serial number or product key. Most are printed on the back-side of the CD sleeve, as sleeve that is usually lost quicker than $20 in Las Vegas. Over at Download Squad, Victor Agreda, Jr. introduces us to a pretty useful utility that pulls out product keys from installed software.
I try to keep my product keys in one centralized location (for me that is SplashID, but there are a few that slip through the cracks. ProduKey does a great job in finding many, although not all, product keys. This makes reinstalls a lot easier.