Cheap Games

Dark Souls I, II, III - They're all pretty cheap now when their on sale.

DooM III BFG - This is DooM 1, 2, tnt, plutonia and 3 all in one game for cheap.

Quake - I reccomend modding this

Duke Nukem 3d - HRP mod.

bullet game

Super meat boy

Wings of Vi

List of vendors that offer steam and non steam PC games for cheap

https://www.humblebundle.com - Charity based games that have been on sale.

https://www.g2a.com/ - Just be aware there is a risk in buying from sellers with a bad rep. The insurance can be worth it. You can also pickup things like Dirt 3 which aren't available on steam here.

Free games

Unreal Tournament - Current dev version is free and it's not bad. Character voices suck other than that pretty good.

Path of Exile - Diablo 3 with a better class and skill system. Hack n Slash. Loot. Crafting.

Strife - The MOBA, not the amazing FPS RPG from the 90s. Which is not free but cheap.

Rumblefighter - Chibi anime version of smash bros. Pretty advanced fighting scrolls based on real world fighting styles like street fighter. This game was really good in the first few seasons way too bloated with uncontrolled game rooms from new exo cores and annoying scrolls. If you play in a controlled environment with guild / friends could still be enjoyable.

World of Warcraft - Free to play level 20 game-play isn't bad and can retain mount and stuff from your old paid account if you create it on the same battle.net

Lineage II - Player killing isn't quite the same anywhere else.

Star Wars The Old Republic - 2011 Sci-fi themed mmo in everyone's favorite galaxy, features graphically impressive space fights that seem less fun than GameCube's Rogue Leader.