Is there a list of GOG games with MOD support and extensive MOD libraries? Or do i have to search by hand one title at a time? Should we add a MODs dedicated sticky in each game's forum?

Also what MOD sites do you people use? Which ones would you recommend?

It depends on the games.

Nexus Mods is usually the best one for alot of games.

Isn't this site more focused on newer games? I'll have to check it out. Thanks

Most games can be "modded", as the term itself is very broad. It could be anything from a translation, or dlc, or skin, or splash screen right up to engine reimplementations and total conversions.
As for sites, there is, in order of preference:
Moddb - the best site for all games. Can be annoying, and sometimes a bit difficult to navigate, but still the best.
Nexus - lots on there and we'll known. I find though that other than elder scrolls games I really don't use the site. Really don't like login for everything, and there is something about the site in general that just doesn't work for me.
After that it starts to disperse into specific sites like doomworld.
One other site which has a bit:
https://www.kirmiziperfect.com/

Lots of other posts on here on this, and look on game specific forums for others. E.g:
https://www.gog.com/forum/general/so_what_are_the_best_game_mods
https://www.gog.com/forum/general/mod_spotlight_bfallout_3_new_vegas_oblivionb_b55c1
https://www.gog.com/forum/general/the_amazing_world_of_neverwinter_nights_mods_26b8a
https://www.gog.com/forum/general/jazz_jackrabbit_mod_spotlight_with_jazz2onlinecom_39cde

Oh, and special mention to taleworlds:
http://forums.taleworlds.com/
One of the most modded games ever M&B. Perisno, prophesy of pendor, brytenwalda to name a few.

In addition to Nexusmods, MODDB is another main one.

Heavengames.com is another one good for old RTS's like Age of Empires, Zeus, etc.

And some games have large enough modding communities that they have their own websites. Eg, doomworld.com (Doom 1-2), neverwintervault.org (Neverwinter Nights), taffersparadise.co.uk (Thief 1-3), etc. So it's also worth searching for game-specific sites like those outside of the "SuperMod" sites like Nexus.

It depends on your defintion of newer games. They have Oblivion, Morrowind, The Witcher 2, Knights Of The Old Republic as well as The Wither 3, Fallout 4 and Skyrim there.

If your after older games then its usually https://www.moddb.com or other fan sites for that game. Theres nothing that has every mod avalible ever. Also you have to remember no every game actually has mods.

Several subforums do have lists, links to and suggested mods.

Thanks to all that are linking sites, subforums and give suggestions. I'll check them out.