Here is a bunch of games for you to consider. Many of these have have sales under 3€:

100% Orange Juice
Accounting VR
Bioshock and Bioshock Inifinite (2K rot them)
Braid
Civilization IV with a A New Dawn mod
Civilization IV with Caveman2Cosmos mod ( I DARE YOU to finish ONE game )
Crusader King 2
Day of the Tentacle
Factorio
Fallout 1 and 2
FTL: Faster than light
GTA: Sand Andreas
Gunpoint
Half Life 1,2, ep1, ep2, and Lost Coast
Heroes of Might and Magic III and V (not the deluxe version of III ! ; is shait! go to GOG)
Just Cause 2
L4D2
Magicka
Mark of the Ninja
Monkey Island 1, 2, and 3
Orcs Must Die 2
Overlord 1 and 2
Painkiller
Papers Please
Portal and Portal 2
Race the Sun
Risk of Rain
Saint Rows 2 and 3
Sanctum 2
Sam and Max all of them except the new reboot version (first one on GOG also)
Sniper Elite V2 (got it for free)
Star Wars Battlefront II (the first one, I mean... well you know)
Star Wars Jedi Knight all of them
Star Wars KOTOR 1 and 2
Star Wars Rebellion (The best grand strategy SW game by far; the oldest also)
Terraria
The Lab VR
The Witcher 3
Tiny & Big in Grandpa's Leftovers
Total War: Rome, Medieval II and Shogun 2
Unepic
Valkiria Chronicles
X-COM 1 and 2 (2K also rot these)