If your browser supports HTML5 you can opt into the experimental HTML5 video playback on YouTube. Not only will you get smoother video playback — goodbye Flash! — but you'll be able to speed up and slow down your videos.
The variable speed control is great for seeing things in slow motion. DIY and tutorial videos often go too fast and watching something in slow motion is usually better than having to watch the same section over and over again to see what is happening. Conversely you can speed up to make finding a section of a long video easier than hopping around from point to point trying to find it.
Visit the link below to opt into the HTML5 beta test. You'll need a browser that supports HTML5 like Chrome or Apple Safari to participate. Check out the link for more details and sound off with your opinion on the new video playback in the comments.