It's Valve. You may know Valve as the maker of highly popular games like Half-Life and Half-Life 2, but Valve's biggest money maker is actually Steam, a site that sells video games for download. Steam sells both Valve's own games and games by other companies, and controls half to 70% of the $4 billion market for downloadable PC games, according to a nice profile of Valve founder Gabe Newell.

The private company doesn't release financials but is "tremendously profitable" according to Newell, with estimated revenues into the hundreds of millions of dollars. And Newell says that Valve has higher profits per employee than Apple or Google.

That's quite impressive. It's also a good reminder that as we focus on social/casual gaming as disrupting traditional gaming, that industry is also going through its own changes.