Viewport

ScrollStory is a jQuery UI widget for building simple, scroll-based pages and interactions. Key features include: 100% style agnostic. Just a collection of often-used scroll-based patterns. Use existing DOM, or use an array of objects to create markup. 16+ jQueryUI-style events/callbacks for various application state events. Focus and blur event when an individual story becomes active or inactive. Items can be grouped into categories, with event dispatched as categories change. Items fil

jQuery-scrollspy is a ScrollSpy library for detecting enter/exit of elements in the viewport when the user scrolls.   jQuery-scrollspy - Features: supports window resize throttles scrollspy events so that event handles only fire every 100 milliseconds  

onScreen is a jQuery plugin that does stuff when the matched elements are visible (as inside the viewport).   The basic form $('element').onScreen(); will only toggle the onScreen class on the matched elements. This is optimal for CSS animations. You can set the tolerance parameter and allow the elements to enter the viewport certain amount of pixels before doing anything.   onScreen also has two methods called doIn() and, obviously, doOut() that will execute when the matched elements enter