Thanks for the nice batch! Added to the list.

Add Far Cry 2 to the list as having a serial key(in library) and it is mentioned on the store page: "Multiplayer notice: In order to access the multiplayer portion of the game, you must first register a Uplay account and enter your unique CD-key. Your CD-key is located on your game shelf."

I haven't tested it as I have no plans to use Uplay. According to this post the game installs PunkBuster(anti-cheat software) and rawmilk905 was able to create a Uplay account and log in but was unable to access multiplayer.

Added!

Besides downloading and checking updated versions of some on your list, the only other thing I have to add for now is...darn it this just seems unfair to add to your list, but it does technically have a serial key! Tales of Maj'eyal has a key to access an online item vault, which when complained about here on its recent release, was patched by the developer instantly to have the vault work offline, without the key(I think I got that right from reading threads? Haven't tested it myself yet)

From what I gathered, the key can still be used for an online version of the vault which can allow item sharing between different computers. At least that's what the developer said. I will add it, but I will appreciate any further info on that matter (whether they key has other uses aside the item vault).

Uhm, why does Don't Starve have a cdkey? It doesn't have MP functions (at least yet). And what does it mean for NWN2 to have one? Is it for MP? Is it attached to some account?

Don't Starve needs the key to enable automatic updates. At least, that's what it says on the product's description.
Neverwinter Nights 2 requires CD keys for the multiplayer, but I don't think it requires an account.

NWN2 has not one, but three keys! lol Hopefully Gragar or someone else can elucidate their purpose as I haven't installed that one yet :)
*hurls shruiken at Gragar, drops a smoke bomb and stumbles back into the shadows*

Wait, it has been a while since i used cdkeys (well, besides using them on gog/steam), if it doesn't stays connected to one account then what would happen if i want to play on my laptop one day and in the other i want to play on my desktop?
I doubt that i'll ever play it online but i'm curious.

I can't be absolutely certain as I haven't personally tested this, but you'll be fine as long as the key is not in use at the same time. If I'm wrong, may the grand Neverwinter Nights Wizard Tallima shed light on this.

Mainly I play those games for it's single player but yeah stalkers online kinda sucks tbh, Everyone I know only plays it for its story and modding single player.

NWN2 has one key. Each of its expansions has another key. Thus 3 keys total.
From what I recall, each player on a server requires a unique serial (or 3, as the case may be), and servers may blacklist keys.
I think they can also totally ignore keys if so set up, but not sure how many servers do that.

I'm downloading ToME now so I'll take a look and see if that key is for anything else.

Installed the latest version of Spellforce(which was different from the old version I have 1.x) and there is no mention of a key being needed for multiplayer. Given the words "GameSpy" at in the background of the multiplayer screen, and I am unable to connect to GameSpy(which no longer exists), I can see the lack of having a server to connect to being an issue. I can however start my own server.
Thanks for the clarification JMich :)

If that's the case, then I assume this is also similar with Dragon Storm and Shadow Wars.

I am curious about something, though; Now that Painkiller has been removed from sale, is it possible to ask a multiplayer key for it from GOG? Searching for the game in support produces no results which means that it's impossible to ask a key for it through the "Technical Issues with games" section.