This is pretty neat:

[TokenHerbz]

Not sure if it's been around for long or not, maybe its something old i came across but...

javascript:var s = document.createElement('script');s.type='text/javascript';document.body.appendChild(s);s.src='http://erkie.github.com/asteroids.min.js';void(0);

post that into your web page. use arrow keys and space bar, enjoy

[Zren]

Yes well, I can make it look like you made a your mom joke with:

javascript:document.body.contentEditable = 'true'; document.designMode='on'; void 0

and ya, that asteroids thing is fun. Especially on your schools site. Midterms? Pfffft. no.

[A.J]

They should create one where you can control the N-game ninja on the webpage. That would be cool.