Copy this:
Code:
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);
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Paste what you just copied, and enjoy. Works for any page. Use the arrows to control your ship, hit space to shoot, and hold B to see all 'enemies' on page.