I haven't created a thread yet (plan on doing so tomorrow), but if somebody wants to beat me to it they can. (If you do, please make sure you make a specific request for Ubisoft to let Cyan buy back all material needed for re-releases of Myst 3 & 4.)

http://forums.ubi.com/forumdisplay.php/191-General-Discussion

Here's a Ubisoft thread:

http://forums.ubi.com/showthread.php/908995-Ubisoft-please-bring-more-games-to-GOG-com-and-let-Cyan-buy-the-rights-to-Myst-3-amp-4?p=10152417#post10152417

And here's a new thread on Capcom. We need more voices there, everyone there asks for Steam, not PC, releases.
http://www.capcom-unity.com/ask_capcom/go/thread/view/7371/30322027/more-games-on-gog-please

Added those! And will be posting in them within the next days!

I don't have high hopes, but nevertheless I made this post on Activision:

https://community.activision.com/message/415342525#415342525

Thanks for it, added!

The Walking Dead games a la Telltale:

http://www.telltalegames.com/community/discussion/79068/please-bring-the-walking-dead-games-to-gog-com?new=1

I'd settle for a patched-up-to-date Sam & Max. :-/

Telltale is added!

Wizards of the Coast, regarding the "Gold Box" AD&D games...

http://community.wizards.com/forum/product-and-general-dd-discussions/threads/4127396

First time I´m adding something non-English:
DeepSilver for Adding MetroRedux!
http://forum.deepsilver.com/forum/showthread.php/68860-Metro-Redux-auch-DRM-frei?p=561463&posted=1
( GermanThread!)

Tecmo-Koei's got a small German/English survey up, feel free to mention that you'd like to see them bring their games to GOG, or at the very least, not make their recent PC games Steam exclusive.
http://www.gamecity.ne.jp/form/gamescom2014

Participated!

Most important right now is NORDIC GAMES, who are pulling their whole catalog from GOG.com:
http://forum.nordicgames.at/showthread.php?186204-Why-will-the-whole-NordicGamesCatalog-be-pulled-off-from-GOG&p=2680511#post2680511 (Darksiders, TitanQuest, BlackMirror, Gothic II& III, countless others...)

Important: This is the INTERNATIONAL part of the forum, so everybody can participate!

There's an explanation here:
http://www.gog.com/forum/general/last_chance_special/post226/?staff=yes
Essentially, neither of them wants this to happen, and they will continue to work to bring the games back here, but there are contractual / legal obstacles that prevent this from happening right now. Nordic and GOG are still on good terms.