Better YouTube + works to improve the YouTube experience by providing quick access to features we'd all like to have from the get-go.

It allows quick thumbnail access of the video.
It allows setting the loop mode easily.
It allows setting the loop mode to a ranged set.
It allows the video to be toggled to fixed or floating with drag functionality.
It allows volume control through the mouse-wheel when hovering over the player.
It allows for quick access to the YouTube video slug.
It allows for NonStop watch mode. (i.e, no "Are you still there?" dialog.)
It shows volume level as you scroll.
It lets Unix, Linux, and MacOS systems have the ability to download the video using native app calls.

Mozzarella

Better Youtube Plus