Any fun games on Steam?

If you're into MilSim check out Squad - it's being actively developed with feedback from the community and feels a lot better than Arma in my opinion.

If you're looking for something to just have fun and dump time into check out Tower Unite - it's basically a port of GMod tower as a standalone game in Unreal Engine 4. Lots of mini games and a casino to waste time at. Fun by yourself or with a group of people.