I think almost any video on YouTube would benefit from having a “play at double-speed” button. Who has time to watch a full 25-second video in real-time?
While I’m (mostly) joking here, this is the kind of thing you’d actually be able to do if the video weren’t in a proprietary Flash player/format – though we don’t have a standard alternative, yet.
That said, the best videos are already only 7-seconds long.
I’d use this feature all the time.