As threatened, Asteroids!
Update: You don't need coins, you can press [f] to start. Apparently I should have made that more clear.
Sorry no sounds, shields or extra weapons,
there are already other good asteroid games for the HTML5 canvas.
I wrote this to try CoffeeScript, which has been quite fun.
I posted the source as a gist.
I have some ideas about how I could make it multiplayer which I might consider trying,
of course that has been done too.