Evenscroll от Steven Lee

Features:
- An additional scrollbar appears on the right side of the webpage you are viewing; drag the slider to scroll the webpage at ease, no matter how long the page is.
- Hover on the slider to scroll the whole screen with your mouse wheel.
- Middle-click on the slider to go to the page bottom.
- Right-click on the slider to navigate back to the page top.
- Double-click on the slider to hide the default scrollbar.
- Hold right-click on the slider to disable.
- Buttery smooth transitions, especially on high refresh rate monitors.
- Movements are interruptible by moving the cursor out of the slider.

* This additional scrollbar only appears on webpages longer than 3 screens height.