I have so many games on my Steam that I won't be able to play them all unless I lose my job and all of my friends.
But here goes...
Cook, Serve, Delicious!
Played this one for 7 hours so far. Lots of fun if you're looking for a quick fix. It's a restaurant game which is a mix of action (preparing meals in real time) and business simulation (upgrading dishes and your restaurant). I only got to the 2 star restaurant so far but it's slowly getting difficult as hell. Picked it up with a Humble Bundle and haven't regretted it.
Game Dev Tycoon
So far I have 29 hours on it. It's a business simulation of a game company with focus on game development and employee management. Lots of fun and addicting as hell. Haven't played it for a while but never regretted getting it during the Steam Summer Sale.
You're the leader of an army in the Middle Ages themed kingdom of Calradia. You can be your own king or fight for one of the other factions. It's open world and the only game of it's kind that I know of, combining RPG, action and management aspects. Definitely worth the money.
Basically DayZ with some sort of semi-magic monsters I guess instead of zombies. Took me 20 minutes to connect to a server and then it crashed on me a few times. I'd rather play an extremely early version of DayZ (which I also own) than this. At least it wasn't expensive during the Summer Sale.
I have long disregarded the Hitman series as just another shooter. I was so wrong. I've only played an hour or so but I can already tell this is a quality game and probably one of a kind. You can get 3 games of the series right now on Humble Bundle.
All that reminds me... I still have to finish Tomb Raider.