Before Valve's efforts a "best of" Linux games list looked like this:
and anything you could get half-ass running with Wine.
I know that's shortchanging it a little, but not by much. Valve has made GREAT strides in Linux gaming.
Although what Valve is doing is great and will probably have a large impact going forward, you can't ignore the humble bundle guys. They have been actively porting games to Linux for quite a while now and if you look at this sale list, almost half of the games on there have been in a humble bundle. Many of which did not have a Linux port before their bundle release.
Good point Arnage. The biggest thing we will be able to thank Valve for after this all comes to be will be that they will have finally been able to get NVIDIA and AMD to give a shi* about their 3D drivers (performance) in Linux (mostly NVIDIA, but the performance of the AMD stuff will also go up a huge degree), since Linux users so far themselves have not been able to do this for the most part. Can't wait!
"Do, or do not. There is no try." - Jedi Master Yoda