Quake III is quite intensive and runs on all platforms. Figure out how it's made.

Not everything needs to be cramped into a browser frame, have an annoying login screen and eat 20% of CPU while sitting idle. Oh, did I mention the hidden benefit of being useful offline?

I'm not really interested in deploying non-web apps at this point.

HTML Canvas element + javascript?

The experience of HTML Canvas element + javascript on IE goes something like this (according to Vladimir Vukićević, the creator of the ActiveX plugin that enables canvas graphics in IE):

"Currently, the experience is pretty crappy: you have to click through an infobar to allow installation of this component, then you have to click 'Yes' to say that you really want to run the native content, and then you have to click 'Yes' again to allow the component to interact with content on the page," he wrote in a blog entry. "In theory, with the right signatures, the right security class implementations, some eye of newt, and a pinch of garlic, it's possible to get things down to a one-time install which would make the component available everywhere."

http://arstechnica.com/news.ars/post/20080819-mozilla-drags-...

If you want to go to war against IE, feel free, but the pragmatist in me says no way.