New all-time high discounts from the "Sci-Fi" games promo,
valid till May 8th:

Cosmic Star Heroine --- 1.19€ (92% off, was 85% before)
AVICII Invector --- 1.99€ (90% off, was 80% before)
AVICII Invector - Magma Track Pack (dlc) --- 0.59€ (90% off, was 70% before)
AVICII Invector - TIM Track Pack (dlc) --- 0.59€ (90% off, was 70% before)
Shortest Trip to Earth --- 5.79€ (77% off, was 75% before)
Shortest Trip to Earth - The Old Enemies (dlc) --- 2.29€ (75% off, was 70% before)
Capes - King City Edition (pack) --- 20.89€ (55% off, was 50% before)

Berserk Boy --- 9.89€ (50% off, was 40% before)
Gori: Cuddly Carnage --- 12.11€ (45% off, was 35% before)
Gori : Cuddly Carnage Special Edition (pack) --- 13.39€ (45% off, was 35% before)
Gori: Cuddly Carnage - Digital Deluxe Edition (pack) --- 15.39€ (45% off, was 38% before)
Lightspeed --- 3.79€ (45% off, was 40% before)
Powerslide --- 3.00€ (45% off, was 40% before**) # best price since Oct.'21
METRO QUESTER --- 11.69€ (40% off, was 35% before)
INAYAH - Life after Gods --- 17.69€ (32% off, was 20% before)

# Ziggurat Interactive
Superhero League of Hoboken --- 1.79€ (70% off, was 65% before)
Slordax: The Unknown Enemy --- 1.99€ (65% off, was 60% before)
Altered Destiny --- 2.59€ (55% off, was 50/55% before**) # first repeat (Oct.'21)
Purple Saturn Day --- 1.85€ (55% off, was 50/55% before**) # first repeat (Oct.'21)
Timequest --- 3.19€ (55% off, was 50/55% before**) # first repeat (Oct.'21)
Zephyr --- 2.59€ (55% off, was 50/55% before**) # first repeat (Oct.'21)
Battle Engine Aquila --- 4.09€ (50% off, was 45/50% before**) # first repeat (Oct.'21)
Wetlands --- 2.89€ (50% off, was 45/50% before**) # first repeat (Oct.'21)
Rendering Ranger: R² [Rewind] --- 7.79€ (20% off, was 10% before)

# DLCs only
X4: Split Vendetta (dlc) --- 4.50€ (70% off, was 60% before)
Distant Worlds 2: Factions - Ikkuro and Dhayut (dlc) --- 5.89€ (40% off, was 30% before)

Thunderful Publisher sale:
Swordship: now -95%; previous high was -90%; Part of Thunderful Publisher sale; Offer ends on: 09/05/2025 09:59 EEST

Not to detract anything from the hard work of the people posting in this thread, but purely to offer an additional resource:
gamesieve: discount quality: improved (excluding games added in the last month), and discount quality: improved, grouping toggled off (so listing expansions, bundles, expansions and goodie packs as separate items).

N.B. I don't track what promo a specific sale belongs to, so find it hard to usefully add recent additions to this thread, but this one is huge:
Tad the Lost Explorer -- $3.99 (-80%; 77% better than previous all-time low: $16.99 (May 2025)) (that previous low price was available until earlier today)

awesome discounts, previously reported by others, thx @ thiagott & gogtrial34987:  
 
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New all-time high discounts from the current "Weekly" sale,
valid till May 8th / 9th:

Redshirt --- 0.98€ (90% off, was 50/60/70/90% before**) # repeat (Feb.'24)
Trine 3: The Artifacts of Power --- 4.43€ (80% off, was 78% before)
Trine 4: The Nightmare Prince (pack) --- 5.99€ (80% off, was 78% before)
Trine 4: Melody of Mystery (dlc) --- 1.59€ (80% off, was 78% before)

 
 
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New all-time high discounts from the Thunderful publisher sale,
valid till May 9th:

Cursed to Golf --- 1.99€ (90% off, was 85% before)
Source of Madness --- 1.99€ (90% off, was 85% before)
Laika: Aged Through Blood --- 9.99€ (50% off, was 40% before)

# DLC only
SteamWorld Heist - Hatbox: Hatful Eight + 2 (dlc) --- 0.59€ (80% off, was 75% before)

 
 
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New all-time high discounts from the "Shooter" games promo,
valid till May 16th:

Tower 57 Two-pack (pack) --- 1.99€ (90% off, was 85% before)
Tiny Troopers: Joint Ops XL --- 1.99€ (80% off, was 60% before)
EPIC + Inferno Bundle --- 1.49€ (70% off, was 65% before)
Neckbreak --- 4.79€ (70% off, was 65% before)
Neckbreak Soundtrack (dlc) --- 0.99€ (80% off, was 70% before)
Tiny Troopers: Global Ops --- 5.99€ (70% off, was 60% before)
Tiny Troopers: Global Ops - Digital Deluxe Bundle (pack) --- 7.49€ (70% off, was 60% before)
Tiny Troopers: Global Ops Official Soundtrack (dlc) --- 1.49€ (70% off, was 60% before)
Tiny Troopers: Global Ops Tiny Tales Comic Book (dlc) --- 0.89€ (70% off, was 60% before)

Atari Recharged Bundle (pack) --- 21.39€ (60% off, was 55% before)
Beat Hazard 3 --- 7.99€ (60% off, was 54% before)
Requiem: Avenging Angel --- 3.69€ (60% off, was 55% before)
Tunguska: The Visitation - Enhanced Edition --- 6.59€ (55% off, was 50% before)
Tunguska: Shadow Master (dlc) --- 2.19€ (45% off, was 30% before)
Tunguska: Dead Zone (dlc) --- 3.29€ (45% off, was 40% before)
Tunguska: Slaughterhouse (dlc) --- 3.29€ (45% off, was 30% before)

Operation Body Count --- 2.89€ (50% off, was 45/50% before**) # first repeat (Oct.'21)
Roboquest Deluxe Edition (pack) --- 18.02€ (40% off, was 30% before)
Speedollama - Game, Art and Music (pack) --- 8.49€ (40% off, was 30% before)
Speedollama - Soundtrack (dlc) --- 2.39€ (40% off, was 30% before)
Speedollama - Art (dlc) --- 1.85€ (38/40% off, was 30% before)
Boiling Point: Road to Hell --- 6.79€ (30% off, was 25% before)

 
 
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New all-time high discounts from the "RPG" promo,
valid till May 16th:

Cthulhu Saves Christmas --- 1.69€ (80% off, was 75% before)
Cthulhu Saves Christmas - Soundtrack (dlc) --- 0.79€ (80% off, was 75% before)
Ruined King: A League of Legends Story (pack) --- 9.00€ (70% off, was 50% before)
Achilles: Legends Untold --- 9.19€ (60% off, was 50% before)
Calico --- 3.99€ (60% off, was 55% before)
The Last Spell --- 10.00€ (60% off, was 50% before)
Dance of Cards - Game + OST Bundle (pack) --- 6.09€ (55% off, was 50% before)
Dance of Cards Soundtrack (dlc) --- 2.29€ (55% off, was 50% before)

Vagrus - The Riven Realms --- 16.54€ (45% off, was 43% before)
Vagrus - The Riven Realms Centurion Edition (pack) --- 25.79€ (45% off, was 44% before)
Vagrus - The Riven Realms: Old Acquaintances (dlc) --- 6.49€ (28% off, was 27% before)
Sail Forth --- 11.09€ (43% off, was 40% before)
Master Key --- 7.19€ (40% off, was 35% before)
Master Key - Bonus content (dlc) --- 2.99€ (40% off, was 35% before)
Master Key Soundtrack (dlc) --- 2.99€ (40% off, was 35% before)
The Iron Oath --- 14.99€ (40% off, was 30% before)
Journey to Kreisia --- 9.79€ (35% off, was 30% before)

Techno Banter: Ultimate Mix (pack) --- 13.99€ (30% off, was 20% before)
Techno Banter --- 13.49€ (25% off, was 20% before)
Drova - Forsaken Kin --- 18.79€ (25% off, was 20% before)
Six Ages 2: Lights Going Out --- 18.39€ (25% off, was 20% before)
Recall: Empty Wishes --- 11.89€ (20% off, was 10% before)
Reignbreaker --- 7.99€ (20% off, was 10% before)
Reignbreaker + Soundtrack (pack) --- 10.29€ (20% off, was 7% before)

# Kagura NSFW
The Triumphant Return of Diabolos (NSFW) --- 8.39€ (50% off, was 25% before)
Magical Girl Celesphonia (NSFW) --- 10.99€ (35% off, was 25% before)

# DLC only
The Edge of Allegoria Soundtrack (dlc) --- 3.19€ (20% off, was 15% before)

 
 
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edit:
Another ATH discount:

Nikoderiko: The Magical World - Director's Cut --- 22.49€ (25% off, not on sale before), valid till May 8th/9th