Bootstrap

If your a web developer you should most definitely be using bootstrap on your projects.
I use it on all websites I work with.

Bootstrap will cut down development times for websites you build which can help with making clients happy.
Bootstrap comes with a load on components which web developers are pretty much guaranteed to use like grid layouts, carousel, cover for homepages, checkout, admin panel and many more great features.

Perhaps one of the best things in my mind is bootstrap has been built with compatibility in mind. one issue we web developers face is making our projects work, function and look correct on all browsers and even older browser versions which may now have some features.
Bootstrap takes care of this for us so we need not worry so much about these issues.

There are also many pre built templates to choose from as a base for your next project.
Bootstrap has way too many cool things to list them all here, so make sure you check it out :)