Z-TYPE
It seems to be a little buggy with Firefox but I haven't noticed any problems with Chrome (which would make sense, considering the whole "Chrome Experiments" thing).
It's pretty fun. Enemies appear at the top of the screen with a word next to them and you have to type that word to destroy them before they reach your ship.
Anyway... the page says that it was made with HTML5 Canvas and JavaScript. This is the first that I've heard of "ImpactJS". Is anyone familiar with it? The game runs very smoothly.