CryEngine, Steam, Unity (engine), LeadWerks, and now Unreal. We're getting closer. This is beautiful :)
I think it really started with the Humble Indie Bundle's mandatory Linux support, accelerated with the help of Unity and Kickstarter, and really achieved mainstream legitimacy with Steam. As long as the GPU manufacturers cooperate and write good drivers, the future looks very bright indeed.
Finally. The year of the Linux desktop is upon us!
Also...you forgot Source.