[UPDATE October 2016] I’ve been using this option pretty much daily ever since posting this in 2013: [Timesaver Tip!] How To Speed Up Online Videos — Offline Too! It’s worked fine in all of the browsers I’ve tried. But recently, in Apple’s Safari browser (all other browsers I’ve tried are fine), the sound muffles slightly when videos are speeded up. The speed boost still works in Safari. I sometimes use it anyway, but for best playback audio quality, browsers other than Safari are preferred. Browsers I’ve tried that have excellent sound quality when speed boosted are Firefox, Chrome and Opera.
HOW TO CHANGE THE PLAYBACK SPEED OF A YOUTUBE VIDEO (tutorial)