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.

Owner of This War of Mine will get 10% off for Frostpunk.

This discount was not in the NEWS thread initially.
Those who are eligible for this offer but didn't get the discount should reach out to Support to resolve the issue.

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.