jQuery is the most popular tools and very essential for every Web Designer and Developer, because its free, lightweight, and it is extendable. there are jQuery plugins freely available for almost every functionality you can think of. Check this top 5 jQuery I listed you might want to check.
Responsive Parallax Scrolling by Cool Kitten
Tool for displaying images made by Fancybox
Video background by Vide
Cool Effects Pop Up Dialog Boxes by Sandra