jQuery.appear is a jQuery plugin for tracking element's appearance in browser viewport. It's licensed under MIT license.
Roll.js is a tiny javasript library to help you keep track of position, scrolling, and pagination.
Scroll Depth is a small Google Analytics plugin that tracks how far users are scrolling. It monitors the 25%, 50%, 75%, and 100% scroll marks, sending a Google Analytics Event at each one.
You can also track when specific elements on the page are scrolled into view.