Updated the first post to make it easier on the eyes. If you spot any mistake or omission, you know what to do!

Point and laugh?

scratch your back ? :P

As long as you can provide the mistake/omission, sure.
Nah, it's not itchy. :P

Added Mount & Blade: Warband - Viking Conquest to the list. According to the release thread, the serials might be slightly delayed.
Edit: GOG has edited the opening post, with no mention of delay for the serials anymore. I should take screenshots next time.

Did Witcher 2 ever have a CD key here on GOG? I checked my spreadsheet and apparently I noted a CD key for it, last updated 2013-08-27. There's no mention on my GOG library. Maybe I made a mistake or it was changed?

Yes. It's included in your receipt of the game. Having said that, Witcher 2's serial is now obsolete (It was once used for a game's registration, but registration shut down some years ago).

Ah, that's it. Thanks!

Added Drox Operative to the list, courtesy of Maxvorstadt.

Not sure if this thread is still used...I'll report it anyway: King Arthur's Gold also has a serial number.

It's still used. I presume that the serial is for the multiplayer?

I'd say it's used for account creation, which I haven't done yet. This is mandatory for the multiplayer part of the game (at least). Singleplayer works without the account.

edit: Attached picture.

Seems to be the case. Thanks for the info. Will add the game very soon.
Edit: Done.

It's been a while since Nordic Games has returned, but I had completely forgotten to update the list to indicate that Painkiller Black Edition, SpellForce Platinum and SpellForce 2: Shadow Wars are now here again, so I fixed it and added the games to the GOGmix as well.