UPDATE
Monkey Island (25%)
Realpolitiks (20%)

holy moly! how do i get the discount?

kingdom rush gets you a 25% discount for kingdom rush frontiers via game code within the account --> game. but it's been cheaper on sale so probably worthless

Just go to the gamecard of Hard Reset Redux, and you will see "You save $17.00 (85%)".

ADDED, thank you!

thanks. i've seen this item act suspiciously in the past. now i know why

I alphabetized the list:

Alien Nations (17%)
Alien Shooter (25%) Zombie Shooter
Alone in the Dark (17%)
Atlantis (23%)
A Hat in Time (13%)
Battle Isle (19%)
Blake Stone (44%) Rise of the Triad
Blitzkrieg (25%)
Botanicula (20%)
Breach & Clear (14%)
Broken Sword (36%)
Bulb Boy (10%)
Cannon Fodder (17%)
Capitalism (19%)
Commandos (19%)
Cook, Serve, Delicious! (20%)
Creatures (17%)
Crusader (17%)
Cultures (25%)
Dark Reign (19%)
Darwinia (Introversion Software) (44%)
Descent (33%)
Deus Ex (35%)
Dungeon Keeper (17%)
Earth 2140 (17%)
Empire Earth (23%)
Eschalon (35%)
Etherlords (42%)
EVERSPACE (25%)
Far Cry (15%)
Finding Paradise (20%)
Freedom Force (38%)
Gabriel Knight (17%)
Galactic Civilizations III (20%) Crusade Expansion Pack
Gorogoa (5%)
Heroes of Might and Magic (36%)
Hotline Miami (20%)
Hybrid Wars (18%)
Imperialism (17%)
Independence war (17%)
Interstate (17%)
Jagged Alliance (55%)
Kim (5%)
Kingdom Come (10%)
King's Bounty (20%)
King's Quest (33%)
Krush Kill 'n Destroy (17%)
Lands of Lore (17%)
The Last Door (10%)
Legacy of Kain (17%)
Legends of Eisenwald (20%)
Legend of Kyrandia (17%)
Leisure Suit Larry (23%)
Little Big Adventure (17%)
Lords of the Realm (17%)
Mages of Mystralia (20%)
Magic Carpet (17%)
Master of Orion (19%)
Metro (25%)
Might and Magic (29%)
Moto Racer (23%)
Myst (21%) Riven, Uru
NecroVision (42%)
Neverwinter Nights (17%)
Niche (10%)
One Unit Whole Blood (17%)
Panzer General (25%)
Perimeter (17%)
Phantasmagoria (19%)
Pixeljunk (24%)
Pizza Tycoon (15%)
Planet Nomads (8%)
Police Quest (25%)
Populous (17%)
Railroad Tycoon (32%)
Realms of Arkania (17%)
Realpolitiks (20%)
Red Faction (25%)
Resonance (Wadjet Eye Games) (20%)
Return to Krondor (17%) Betrayal at Krondor
Revolution 1979 (33%)
Samorost (15%)
Sam & Max (58%)
Satellite Reign (13%)
Serious Sam (33%)
Seven Kingdoms (17%)
Simon the Sorcerer (22%)
Space Quest (25%)
S.T.A.L.K.E.R. (20%)
Star Control (17%)
Still Life (25%) Post Mortem
StrongHold (23%) Space Colony
Syndicate (17%)
Tex Murphy (31%)
The Escapists (31%)
The Final Station (10%)
The Longest Journey (15%)
The Lords of Midnight (17%) Doomdark's Revenge
The Secret of Monkey Island (25%)
The Zork Anthology (17%)
Thief (37%)
Tomb Raider (33%)
Tom Clancy's (33%)
Total Annihilation (17%)
Treasure Adventure World (20%)
Turok (25%)
Ultima (17%)
Unreal (40%)
Uprising (17%)
Victor Vran (15%)
Warhammer 40,000: Armageddon (29%)
Warhammer 40,000: Sanctus Reach (15%)
Warlords Battlecry (18%)
What Remains of Edith Finch (5%)
Wing Commander (27%)
X-COM (50%)

REMOVE
Simon the Sorcerer (was 22%)

This change is due to the release of 25th Anniversary Editions.

But note, Simon 3D still has the 22% buy series discount if you own the other removed 2.

UPDATED, thank you!

Owner of legacy Simon the Sorcerer 1 or 2 will get 50% off for their 25th Anniversary Edition.

I am not sure if it is time-limit or not, so I will not edit OP for now.

REMOVE
Pizza Tycoon (was 15%)

This change could be a bug, but I have no plan to submit a ticket about it.

vidsgame found a new series discount for Crysis.

ADDED, thank you both!

Some discounts quoted above are not working now.
I have filed an GOG Support ticket for it, #248383.

Discount for Simon the Sorcerer 1 & 2 25th Anniversary Edition is still working.

REMOVE
Discount for Frostpunk