Custom Video Speed for YouTube

Custom Video Speed for YouTube is a Safari extension that lets you have much finer and more consistent control over the playback speed of YouTube videos.
The extension automatically applies your current speed as you click through new videos and you can set your current playback speed as the default so that every time you open a video, it is automatically set to your preferred speed.
You can control the speed of YouTube videos using the pop-over on the extension or using keybindings on the keyboard. These keybindings can be customized in the options page or turned off all together if you don't like them.
You also get finer control over the speed than is provided by the YouTube player. You can move in increments of 0.05x 0.1x, 0.5x, and 1.0x. You can also go from a little as 0.1x normal speed all the way up to 8.0x normal speed.