I wish I had kept an eye on it after it was announced (which was 3.5 years ago, but not very stable): The Youtube HTML5 viewer trial.
It has come a long way since then. Full screen it is still not as good as the official one, but the main attraction introduced since is: configurable playback speed!
Yes, you can choose playback at 25%, 50%, 100%, 150% or 200% of the original speed. Ideal for lectures or watching replays of conference sessions.
Quote from the life hacker post LifeHacker on this:
The Wiert Corner - irregular stream of stuff 