Just curious, how are they able to do that ?

[Impert]

http://www.adfactorypro.com/examples/beforeyouleave.html
Without reverse engineering their code (they are selling it in a software and it would not be fair), do you know where I could find something similar (or not far) for free ?
Thanks

[BLiZZaRD]

Using Javascript, you can get the X and Y of the mouse, use the mousemove() function and declare a pop up when mouse moves to a certain XY co-ord.

A simple breakdown of mousemove can be seen here (http://www.w3schools.com/jsref/jsref_onmousemove.asp)

Search google. There are things you need to know about mousemove that will help, such as turning it on only when needed and off when not (null) this will save system resources as mice move A LOT!

[boogyman]

without looking at the code, it is probably checking to see the position of the mouse and when it gets to a certain point in the viewport (part of the browser you see the website in) it displays the popup.