A full 3D engine, with dynamic lighting, textures, bump mapping…written in Flash.
I remember reading a while back that Quake II had successfully been ported to Java, 8 years after its initial release (although it still required you to have the game data files on your hard disk, naturally). Some time afterwards, id software announced that it was working on a version of Quake III Arena you could play in your browser. I was skeptical about it at the time (and the fact that the game actually launches from your browser but runs as its own executable appeared to have borne out my skepticism), but given this particularly impressive tech demo, it may be entirely possible to achieve.
Granted, the graphics are barely up to the standard of 1998, and those textures are rather blurry, but they\’ve got to start somewhere.