High five to the Greensock gang for the ScrollTrigger release. The point of this new plugin is triggering animation when a page scrolls to certain positions, as well as when certain elements are in the viewport. Anything you’d want configurable about it, is. There’s been plenty of scroll-position libraries over the years, but Greensock has a knack for getting the APIs and performance just right — not to mention that because what you want is to trigger animations, now you’ve got Greensock at your fingertips making sure you’re in good hands. It’s tightly integrated with all the other animation possibilities of GSAP (e.g. animating a timeline based on scroll position).
CodePen is full of examples too, so I’ll take the opportunity to drop some here for your viewing pleasure. You can play with it on CodePen for free (search for it).