This morning I found a nifty bookmarklet at http://erkie.github.com/ that allows you to turn any website into an old-school game of Asteroids.
You control the little ship using the arrow keys, space bar to fire, and shoot at the various elements of the page – which disappear as you hit them.
I recorded a screencast using spudart.org as an example. It's a bit difficult to make out the ship. But if you change the video resolution to 720, and view it in full screen, it's a little better.
(If you click here, you can try it out on my blog. *gasp*)