An updated list:
Overload - Not on sale at all, but totally worth it! :)
Beyond Divinity - Now I own all Divinity games except for Original Sin 2.
XCOM: Enemy Unkown Super Duper Ultra Totally Complete Pack - Damn skippy!
Into the Breach - I was going to hold off until the Mac version released, but I started to feel silly knowing I had a perfectly good Winbox I could play this on. It probably works in Wine, too, but I'm not sure I'll bother.

I'm still debating whether to pick up Divinity: Original Sin 2. I know I won't play it at least until the upgraded definitive edition releases so I'm not sure whether buying right now is particularly worth it. *shrug*

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X:COM Enemy Unknown Complete Pack
Oriental Empires
Unlocked Sunless Sea and Rime
Ziggurat (Freebie, btw thanks again GOG)
Legend of Grimrock (GOG Connect)
Starpoint Gemini 2 (GOG connect)
Prison Architect (GOG Connect)

Nothing so far. I'm having difficulty thinking of something I'd actually have time to play, even though I'm all over buying at least $5 worth to get my hands on Sunless Sea. Though Sunless Sea is on sale for $6.49, so that would be enough to get the unlock...

Oh, wait.

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So far, I bought Night In The Woods and Doom 3 BFG, so I got the 2 free games as well. Maybe I'll buy Hyper Light Drifter.

The Shivah (too short for my liking)
Technobabylon (getting ready for Cyberpunk 2077)
Theme Hospital
Divinity: Original Sin
Ultima 8 Gold Edition
Grim Dawn - Ashes of Malmouth (the new class, Necromancer, is so much fun)

Sunless Sea: Zubmariner (as a gift, to unlock the base game :D )
Baldur's Gate: Siege of Dragonspear (with the bundle discount it was too cheap to pass)
Atlantis: The Lost Tales (with fair price money I got back)
Atlantis 3: The New World (with fair price money I got back)

Sunless Sea (freebie, unlocked with Zubmariner :D )
Xenonauts (freebie)
Ziggurat (freebie)
Kingdom: New Lands (connect)
Psychonauts (connect)

I decided to stop by and do some shopping since I was slightly curious about Real Myst. I hate re-releases, so let's buy it... I'm not sure about this logic though.
I don't know if I will care about it, I don't know anything about Myst. Well, there it is anyway. Now.

Since I was shopping, and I've been thinking about trains for a while, I slightly looked at few reviews of few games and bought Railroad Tycoon 3 after that.

And since I was here, shopping, and I have wanted the Lion King for a while I got that too even if it is not less than four euro.
I'll get the other two Disney games someday once they are very cheap, so it will all add up to less than four euro/game. That sounded like a plan to me at the time.

XCom: Enemy Unknown
Ember
AER
Alpha Centauri (my CD copy doesn't have Alien Crossfire, and the discount got high enough)
Myst V: End of Ages (I somehow missed playing it in the early 2000s)

...and the freebies I didn't already own.

Sticking tight to my budget, already playing Ember, and planning to dip into Myst V tonight. Got my husband his hassle-free Father's Day gift (XCom).

Mafia 2 DC
Mafia 3 Deluxe Edition

Free Games
Ziggurat
Sunless Sea
RiME

Already posted it in another thread but here we go:

Real Myst
Enclave
Oddworld: Stranger's Wrath HD
Oddworld: Munch's Odysee
The Park
Downwell
Hyper Light Drifter
AER - Memories of Old
Blackhole
The Saboteur
LEGO: Pirates of the Caribbean
LEGO Indiana Jones: The Original Adventures
LEGO Indiana Jones 2: The Adventure Continues
Higurashi: When they Cry Chapter 1
Ruiner

(Ziggurat and Sunless Sea were given for free. I already owned RIME and gifted it)
Who said this? All videos I saw of it ran flawlessly - and the game is not THAT old.

Hope you did not forget 400Days if you want to complete your TWD collection.

New purchases during sale:

SOMA Deeply discounted and from Frictional Games. Easy decision.
Crusader Kings Complete I really enjoyed "Castles 2: Siege and Conquest" back in the day (also on GOG), and this game sort of looks like a more sophisticated, polished version of that.
Ultimate General: Gettysburg (big fan of the American Civil War although I'm not a big strategy gamer. For $2.29 it's a no-brainer though. If I end up enjoying it I'll look to "Ultimate General: Civil War" in the future).
Tyranny (base game with Tales from the Tiers DLC, from others' recomendation). Big fan of old-school Bioware/Black Isle; lukewarm fan of "Pillars of Eternity." Some critics say this is a more nuanced cRPG with a superior story, so what the hell, it's on sale for the most it'll probably be for a long time.
Walking Dead: A New Frontier (already picked up Seasons 1/2 on an earlier sale; this one finally came significantly down in price).

And I claimed Ziggurat for free, and like others got Sunless Sea and RIME free with my purchases.

With GOG I really don't need any other gaming outlet.

Unreal Tournament GOTY
UT2004
System Shock Enhanced Edition
System Shock (it comes with Enhanced Edition)