Plugins are simply tools which work with the WordPress software to extend its core functionality. For WordPress, plugins are essentially small PHP scripts which serves certain set of functions which can be used to enhance the site functionality.
There are hundreds of thousands of WordPress plugins, for nearly every imaginable purpose. But once you find the functionality you want or need, it becomes even more challenging – you also must know how to use those WordPress plugins and tasks such as install, configure and deploy. The various plugin can be viewed in WordPress official repo https://wordpress.org/plugins/
There are two ways to access or download plugins and install them on your WordPress system – directly from the Plugins page which is available through the administration dashboard or by using some file transfer protocol (FTP) to upload to the appropriate folders on your Web server.
Some of the frequently used plugins:
Contact Form 7
Yoast WordPress SEO Plugin
W3 Total Cache
Js_composer ( Visual Composer )
Mailchimp For WP
Commercial plugin marketplace :
CodeCanyon is another marketplace by Envato. If you’re looking to sell WordPress plugins, CodeCanyon is a good candidate for your products.
Mojo has another marketplace for scripts and plugins called Mojo Code.
WP Eden allows other to upload and sell WordPress plugins on their growing marketplace.
wedevs creates the awesome plugins and showcases for the sells.