What are the best free games for a low end pc?
Duelyst II is a fan revival of essentially a Hearthstone clone played on a chess board. It's still in it's early stages, but it's fun and promising.
Muck is a roguelike survival. It's got kind of a silly TWAB vibe. It's fun to play with friends but you don't have to.
TF2 is a casual FPS, as you probably know. It still holds up. I'd recommend playing on custom servers.