Good old Valve. Now I'm not someone you'll ever hear fawning over a company or its products, maybe it's the generation I come from or just my inner idealist. A little mistrust of capitalist enterprise is healthy even though capitalism is way better than either of the two big statist experiments of the 20th Century i.e. communism and fascism. Still, it's not perfect. The bottom line is that companies exist by exploiting their customers for profit, and as a customer, it's wrong or even foolish to expect more.
Sorry about that, went off on one a bit there. Where was I? Oh that's right, good old Valve. They've released Steam for Linux - well, it's currently in public Beta, so it's available to all. Only Ubuntu is officially supported, but I've had no problems running it in Arch using the package from AUR.
There's nowhere near as many games available compared to what you're used to with Steam in Windows: the Linux section of the Steam store shows a total of 42 games. Valve's own free-to-play Team Fortress 2 is the highlight, and it runs perfectly with silky smooth graphics. Of course, your hardware has to be up to the job: for some reason I'm not able to switch to any resolution other than my monitor's native 1920x1200. Because of this, the game does get jerky at times on certain maps, and I've just today ordered a new AMD Phenom II X4 965 CPU, to replace the Athlon X2 4800+ that has served me well for the last 2 or 3 years.
So far, I've bought three new games all of which run totally fine on my system: