UhuruNUru: GOG uses no CD Keys for single player games
Maighstir: Some of them actually do. Except since the key's only checked locally (meaning it has to follow a certain pattern, not necessarily being unique), it's quite possible to use the same key for all installations, and that key is set up by default by GOG's installation and only needs to be switched for a unique key if you want to play multiplayer.
Example: Neverwinter Nights.
You could argue that it's not DRM, since the key is installed automatically (bundled in the installation) and is not unique, but it's still a CD key that would have been entered manually in non-GOG editions, and the (local) check for a valid key has not been removed.
Well I'm arguing AllCD keys are DRM so I'm not going to make an argument the opposite but all cases I know of on GOG that have a CD key are for the online component of a game. Howrver the exact mechanics work will depend on the system implemented for each game. The basic fact remains it's for online verification not offline gaming, with some games these are likely unable to be separated.
There will always be an exception due to variations in how specific keys work but the core reason of being for online access is the why any CD key for games on GOG.
This is all a side issue to my reason for mentioning CD keys in this thread though, in an effort to get back to that unresolved issue I will try again.
Some games listed as DRM Free on Steam in this thread, do require a CD key (Fallout 3 does) when copied after applying that key (often done automatically by Steam) the game should run fine without said key. It should be noted in any list of games that run without Steam if a CD Key is used and a recommendation to record the Key from the Steam Client.
Knowing a key may be required can save a lot of hassle if it ever gets corrupted or deleted from the registry. IF Steam does ever cease to exist it will be to late to get the key then.