My most used bookmark is this, for speeding up the first audio or video element on the page. I suppose that's if you don't count me typing 'n' and auto-completing my way to hacker news.

    javascript:(function() { (document.querySelector('video') || document.querySelector('audio')).playbackRate = (+prompt('How many times normal speed?') || 1); })();

For webkit based browsers, you can also use Video Speed Controller extension

https://github.com/igrigorik/videospeed