I've created GOGmix with list of old games which still have an active modding community: https://www.gog.com/mix/old_games_with_new_mods

There are some which I know, if you have idea for more, I will update the list.

By the way, is this page feature sort of dead? I can't find how I can get back to my list, other than keeping url of it.

Baldur's Gate.

In fact all Infinity Engine games, so IWD and PST too.

e.g.:

The Gibberlings 3:
http://www.gibberlings3.net/

Pocket Plane Group:
http://www.pocketplane.net

Spellhold Studios
http://www.shsforums.net/

Mount and Blade games: not sure if all of them do but at least Warband has current mods

Stalker series

Morrowind.

There are a number of mod-related websites, the most important one being Nexusmods (http://www.nexusmods.com/morrowind/?), but also Morrowind Modding History for older ones from long-dead websites.

Neverwinter Nights still has mods popping up now and then.

Does Crysis count as old ? Well it's 10 years old and it should.

There is a pretty good mod released just few months ago, it's called Triptych. It's a very atmospheric Lovecraft based "walking simulator" with very high production values. Highly recommended.

http://www.moddb.com/mods/triptych

Deus Ex and Doom

Both Civilization III and IV have a a vibrant modding community.

https://forums.civfanatics.com/forums/civ3-creation-customization.46/

https://forums.civfanatics.com/forums/civ4-creation-customization.158/

It's been like that for ages. Plus the votes don't seem to be counted properly.

Great idea for a gogmix. Depending on how you define a mod there are several that can be added:

openrct 2 for RCT2
OpenXcom for Xcom:UFO Defense
v1.13 for JA2
Restoration Project for Fallout 2

Thank you all for you suggestions! How could I forget all these games? List is updated.

Some games are just not old enough (Warband and Crysis) for my personal taste, but do not despair! They will get there eventually!

If you like the mix idea, would you be also so kind to upvote the mix, for better visibility?

Enemy Engaged: Comanche vs Hokum
http://simhq.com/forum/ubbthreads.php/forums/13/1/EECH_EEAH.html

Note that latest version of the mod not longer follow the spirit of the original game, and convert it in a simulator only for purists.

Some games to add.

Alpha Centauri
http://www.alphacentauriredux.com/

Star Wars: X-wing Alliance
http://www.darksaber.xwaupgrade.com/

Freespace 2
http://www.fsoinstaller.com/

Archimedean Dynasty
http://www.moddb.com/mods/archimedean-dynasty-augmented-mod

+1 To Nightblair for this great mix.

I have added the games to the list. Thank you for suggestions!

Unfortunately the Alpha Centauri UI Redux is only a design project, not actually a functional mod.