ViewportDetector.js is a lightweight and convenient JavaScript library to detect if an element is inside the viewport and allows you to do some stuff as the element becomes visible on scroll.
hunt.js is a lightweight yet robust JavaScript library used for the detection of HTML elements within the browser viewport.
enter-view.js is a pure JavaScript library that for the detection of HTML elements within the browser viewport.
nowUseeMe is a Vanilla JavaScript solution to detect if an element is visible within the browser viewport. Also supports window scroll events.
white-in-view is a very small JavaScript library that checks if an element is in the viewport and adds matched CSS classes to it based on the current in or out state.
Just another JavaScript library which detects when an element is in the viewport and triggers custom events when it is scrolled into or out of the view.
inView.js is a pure JavaScript library that watches for the scroll event and executes callbacks when an element is scrolled in or out of the viewport.