Borderlands, The Witcher 2, Metro Redux, XCOM, any Valve games. My point is that the number is much better than it was just a year ago and will get even better as more steam machines are released.
I personally do not expect Steam Machines are going to do much of anything. We have no evidence that will say they will, I am interested in them because Steam Machines run Debian at their heart and I love Debian. But right now (native) Linux gaming is still a joke and there is no evidence that will change in the near future. As I said I run pretty much any indie game I want using Wine on Ubuntu. Indie games and strategy games (with occassional RPGs) is all I play on PC but I am interested to see if Steam OS and the Steam Controller can bring strategy games to the big screen! I think the Steam Controller is going to succeed but I think Steam Boxes and Steam OS are going to flop hard.