Jquery-scroll-events is a project mainly written in JavaScript, it's free.
Special Scrolling Events - Useful for scroll pagination
JQuery Scroll Events
This plugin was created to create special events to the scrolling actions. See details browsing http://code.google.com/p/jquery-scroll-events/
Licensed under MIT License. Details: http://www.opensource.org/licenses/mit-license.php
Just include the Javascript file after you've included jQuery.
Something like this:
Complete Usage details can be found at http://code.google.com/p/jquery-scroll-events/wiki/Reference