This script is good..
If u like to read thing and do work on the internet, u must have this

I'd love to have this same thing when reading PDFs, eBooks, etc.

It helps quite a bit since it can be confusing when you switch programs/operating systems/etc. and the amount each scrolls varies.

At least know in Firefox I'll be able to quickly find my place!

It works no matter how you scroll, whether it's with the mouse, arrow keys, or spacebar.

Edit: Thanks TekZilla for pointing this out.

As a stock market trader I skim-read a lot of web pages every day for information, and as I'm scrolling quickly through a page with the mouse wheel I often lose where I am momentarily. With this script installed, a thin pink transparent line is placed across the page at the bottom of the visible screen, so that when you scroll down, the strip marks the point you read up to. It's a simple thing, but incredibly useful.

I've found it's not perfect - on certain pages the pink line is left hanging further up the page rather than resetting downwards - I believe this is due to a conflict with NoScript when some scripts are blocked. But I'd still rate this 5/5.