For most paid classics there is a free, open-source equivalent that is genuinely good — not a knock-off. Every recommendation here has cleared the PlayBound Bar.
Command & Conquer
3 free options · also Red Alert
Age of Empires
3 free options · also Age of Empires II
Supreme Commander
3 free options · also Total Annihilation
Cities: Skylines
3 free options · also Transport Tycoon
Minecraft
3 free options · also Minecraft Java Edition
Factorio
3 free options · also Satisfactory
Worms
2 free options · also Worms Armageddon
Mario Kart
2 free options · also Mario Kart 8
Quake & Unreal Tournament
3 free options · also Quake III Arena
Diablo
3 free options · also Diablo IV
Elite Dangerous
2 free options · also Escape Velocity
Super Mario Bros.
2 free options · also Super Mario World
Civilization
3 free options · also Civ 6
Microsoft Flight Simulator
1 free option · also MSFS
World of Warcraft
4 free options · also WoW
Destiny 2
2 free options · also looter shooter
Transport Tycoon
2 free options · also Transport Tycoon Deluxe
Fire Emblem
1 free option · also Advance Wars
Need for Speed
2 free options · also Forza Horizon
Mega Man
2 free options · also Mega Man 11
Super Smash Bros.
3 free options · also Super Smash Bros. Ultimate
FIFA & Sensible Soccer
2 free options · also FIFA
Super Bomberman
2 free options · also Bomberman
Axis & Allies
3 free options · also Hearts of Iron IV
Wing Commander & Freelancer
4 free options · also Privateer
Sega Rally & Colin McRae
2 free options · also Colin McRae Rally
Gradius & R-Type
2 free options · also Gradius
Overwatch 2
3 free options · also Overwatch
Diablo IV
3 free options · also Diablo 4
Baldur's Gate 3
3 free options · also BG3