Windows product keys can be defaulted during installation (meaning that the user doesn't have to rummage around for the manual/sticker while installing; they can just press enter) by creating a file named msbatch.inf in the setup directory, containing
Signature = "$CHICAGO$"

This will work for windows 9x, and presumably for any product that allows administrative install. This way you can backup your legitimately purchased product key at the same time as your legitimately purchased install media
Obviously, you'd want to use a real product key... The legendary 00000-000-0000000-00000 key doesn't work after OSR2.

The product key authentication in Quake 3's interface was quickly reverse-engineered to make key generators. Unfortunately id thought of this; The algorithm has a few billion valid keys, yet only the few hundres thousand keys id have shipped are authorised by the server (This would be why product-key checking is not optional). Your chances of generating a quake3 key that works on the internet are slightly lower than winning the lottery.