There's tons of free to play titles on steam that are worth your time. 1. Warframe: Play as a space ninja in a very well rounded and expansive world consisting of procedurally generated maps, varying enemy factions, PvP, tons of weapons and character building opportunities, and an incredibly rich story and lore background. Little Grundy at times but shits worth a try. 2. Path of Exile: Think Diablo but ginormous and constantly being update. I am just getting into this game again and there is so many different builds and approaches to this game it's almost overwhelming. I have never seen a game with a bigger, crazier skill tree than this that lets you truly customize how you wanna play. Worth a look. 3. Robocraft: This'll be hit or miss with people, but I get ceaseless entertainment out of this game. A complex building system allows you to construct planes, tanks, helicopters, cars, rocket sleds, and pretty much fuck all else and then put weapons on em to fight people. I like it because the combat feels good and I get to really exercise the creative nerd in me to design and test shit. If your into shit like that, would recommend. 4. Team Fortress 2: Everyone knows tf2. Shits still fun as fuck. I still play now and then. Great team based fps. It's free. No reason not to try.

Prolly more I'm forgetting but those are some free games that are close to my heart.