All of a sudden, Valve has a monopoly on a burgeoning Mac game market. Steam is being released on OS X in April, accompanied by Valve's back catalog. If you already own a Valve game on Steam, say Half-Life 2, you can install Steam on your Mac and download it free of charge. All future Valve games will be dual-platform releases, available immediately for the Mac along with the usual Windows version. Portal 2 will be the first dual-platform release in November. This also means that Mac versions of games can play with Windows versions of games. If you're playing L4D2 on the PC, half of your group might be Mac players while the other half are still on Windows. Lastly, Valve is going to help the various third parties who sell their games on Steam to create native Mac versions. This means that Valve is more or less the de facto largest Mac game publisher on the planet. I would imagine they could make an absolute killing licensing the Source engine now. 2010 is a pretty freaking awesome year for video games thus far.