[sralbert]

This is HN so if by "free" you mean open-source ignore this.

Rocket League

Counter-Strike: Global Offensive

Team Fortress 2

Starcraft 2

Fortnite

Diabotical

[tubularhells]

TF2 is great. Don't play causal matches, but find a community server where aimhacking and botting is not allowed, though.