Recently I ran across Rawkets, a slick site taking two emerging web technologies -- HTML5 Canvas and WebSockets -- and combining them in the most obvious way possible: a multiplayer space shooter.
Why Canvas? No plugins -- graphical Yes; and why WebSockets? Low latency -- multiplayer Yes.
Sadly, every time I join the game, nobody else is there. If I wanted single-player HTML5 gaming, I could check out another project by Rawkets' creator, Rob Hawkes: straight-up Asteroids, using the HTML5 game engine Impact.
But WebSockets won't help Asteroids, because Asteroids runs totally on just one client.
Check it out, and maybe shoot someone else's ship down -- fairly fairly, of course, because WebSockets will keep multiplex channels persistently open...