[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, I no longer use it in Apple’s Safari browser because the sound degrades when boosting the speed. So for now, at least, I recommend using this in browsers other than Safari. It still works in Safari, but the sound isn’t as clear.
HOW TO CHANGE THE PLAYBACK SPEED OF A YOUTUBE VIDEO (tutorial)