How do I know which games require these clunky keys for multiplayer and/or other things? I downloaded Two Worlds: Epic Edition, and it has this whole key business going on, but I don't see anything on its GOG page indicating this.

Is there a list of key'd games?

No list that I know of and as far as I know, the only way to see if a key is needed is to dig around in support..

Not the handiest I know, but I think they are working to automate the key process...

EDIT: I don't see anything about Two Worlds needing a key either...
EDIt 2: I don't have any key for it....

Doom is all about keys. :D

I know the Far Cry games require them for multiplayer.

The gogwiki has a category for these games, but I'm not sure how complete it is.

[url=http://www.gogwiki.com/wiki/Category:Games_with_multiplayer_keys]http://www.gogwiki.com/wiki/Category:Games_with_multiplayer_keys[/url]

Normally the page mentions the key.:
Earth 2160: "Product activation notice: You will be able to play the full single-player game without using the "Product activation" button in the main menu. This function is only required for online play. To receive a multiplayer key, please contact Support. For details click here."

American Conquest: "Multiplayer notice: multiplayer mode is available after registering your unique CD-key for the game. For details see: Support."

etc

It is missing the recent releases ... at least I don't see Heroes 5

Heroes 5 doesn't mention needing a key even on this site... at least not that I see... :/

I downloaded it to play the SP and after a bit, I saw that it had a coop option. All the GOG stuff (except the then-free Empire Earth) I've downloaded from GOG for SP only. So I tried to install the game on my other PC just to try out the multiplayer, and I came across this whole key thing. Searching the forum led me to emailing support for a key, and they were fairly prompt at giving me a key.

Are you sure that you don't need a key to play the mutiplayer mode? I just went back into my install and, as the vaguely Nightwish-sounding music played in the background, I clicked on "multiplayer" and I was given a rather austere screen mandating that I enter a key (I haven't entered my key yet).

The whole concept of this key doesn't make sense to me. It says that it is a-ok to play 2 separate instances of a single player game on 2 separate computers, but that it is 100% not cool to try and combine those instances in a coop game. Being an old-school PC gamer who remembers the days of Age of Empires II, Starcraft, Freelancer, Tribes II and the like where the games were set up to allow LAN play with limited physical copies (but restricting online play), I found this to be pretty jarring. I'd like to stay far, far, far away from this in the future.

The only option to play a game with keys in LAN coop is for me to make myself a second account, somehow, and then rebuy the game. It all just seems so unnecessary and difficult. If GOG feels compelled to have this semi-ironic key system in place, I'd appreciate it if they'd have some sort of setup to allow me to buy multiple keys from a single GOG account.

IIRC sometimes the key field will be there but accept any key at all, valid or otherwise as the code jumps around it. I'm not sure which games that might apply to here.

I hadn't seen that before. I don't remember requesting the keys, so have they implemented automatic keys for some games already?

They were there ... I guess it is for newer releases. And if I recall correctly TET or some blue one told in one thread that there will be an automatic system for keys.

Say, if this was announced somewhere else I missed it, but why are there keys on GOG games in the first place? It just seems so ironic that I'm still shocked and in a bit of disbelief that they're even here. Did GOG reevaluate its mission or institute some other "paradigm shift" and I just missed it? Again, I don't want to ever get another game which has these keys as they don't even make sense.

They are there maybe cause many of those games that have it are mainly single player games. Or they are very old that they can't remove it.

They said that there's only one key given out per GOG account. That makes me think that GOG endorses this wonky DRM. They'll give out the single player version of a game, a game which is all about single player, with no restrictions, but they'll put the kibosh on the included coop mode? I just can't see the logic in it. It's like they'll give us complete freedom with 95% of the game, but then go all totalitarian regime on us with the last 5%.

I mean, I'd buy another copy of the game to do try out this coop mode. I wouldn't be smiling, but I'd do it. GOG doesn't provide for this, which seems like an oversight. I don't see how the key system is not DRM, but it's still better than Steam's DRM so I'd go for it. I could try and make another account, but I don't know if GOG has anything in place to try and stop that. I could have my friend create a GOG account for this, but GOG is supposed to save us from that Steam-like foolishness.

This whole encounter has made me skeptical of GOG. I guess this has been going on for a while now, but playing most all of my games from here in SP I guess I didn't notice.

Try the search function, there have been several threads about this already.