Bootstrap is a free and open-source CSS framework directed at responsive, mobile-first front-end web development. It contains CSS and JavaScript-based design templates for typography, forms, buttons, navigation, and other interface components.
Jetpack is a popular WordPress plugin created by Automattic, the people behind WordPress.com.
OWL Carousel is an enabled jQuery plugin that lets you create responsive carousel sliders.
Slider Revolution is a flexible and highly customisable slider.
WordPress is a free and open-source content management system written in PHP and paired with a MySQL or MariaDB database. Features include a plugin architecture and a template system.
WPBakery is a drag and drop visual page builder plugin for WordPress.
Yoast SEO is a search engine optimisation plugin for WordPress and other platforms.