This useful extension contains a script which replaces the standard Flash based videos on Youtube with the new HTML5 video tag. These video will require much less CPU power. Not only using less CPU power, it automatically attempts to play in the best possible format (HD if available). The only down side to this extension so far is that any videos that are embedded on sites other then Youtube will still use the old Flash based player, and annotations will not be displayed for any videos. However, this extension is still in development, and the developer is currently working on these issues.
To download this extension, visit: Youtube HTML5-ifier