YouTube for Musicians от MusicLessons.com

This extension modifies YouTube.com video views by adding a musician-friendly video controller directly under the video. The controller allows the user to access standard play/pause/ff/rw controls, as well as controls that create and playback loops.

The top slider shows video progress and can be used as a scrub control.

The bottom slider is the loop control, where the left handle is the start of the loop, and the right handle is the end of the loop. There are button controls above both of the sliders that allow the user to enable/disable the loop, as well as set a start point and end point.