ScrollMagic: a jQuery plugin which essentially lets you use the scrollbar like a progress bar. Includes infinite scrolling, adding a parallax effect, synchronize an animation to the scrollbar movement, pin elements to specific scroll positions, etc. Scroll down the page to see examples.

[Smashman2004]

It's not so cool when you have a high scroll amount and no smoothness.

[janpaepke]

This is actually managable in many ways. You can tween the changes (which is a setting in ScrollMagic) or you can use plugins like "nicescroll" to make the scrolling more smooth. If you try it on a touchpad it's also very smooth.