Immersive content for the Browser
Second life is probably the best example of a 3D simulated universe, a highly immersive experience.
Adobe Flash is used by pretty much everyone in the 2D gaming community. Check out Club Penguin and Habbo as examples of this approach. Unity 3D is also available for doing Browser-based games in Java for the web, and also for a raft of different devices.
All in all, developers love it when they can target different devices and form-factors from the same code base, without worrying about obsolescence. HTML 5 may really shine there.