Definitely have a look at some of Valve games! Portal 2 is amazing and other than being in first person isn't really a shooter at all, just a puzzle game with a stunningly told story. You might also want to give TF2 a try, it is a shooter but a very unique one (lots of different characters for different styles). it's also free, so no harm trying :P
If you haven't already played it yet, try out Skyrim. Having access to all the mods really makes it the best version of the game by a mile.
If you like strategy Civ 5 is a brilliant way to waist several hours/decades of your life. Starcraft 2 is also fantastic, but it's not available on steam (battle.net is still great for what it does though). The Total War games are also very good ^^
Also try out Amnesia for a great puzzle game :) It's very calming to play if you have a pants shitting fetish
