Some of the best free WordPress plugins are as below.

Truepush: to send free push notifications to re-engage and retain your site visitors.  
Yoast SEO: to optimize your site’s on-page SEO parameters. 
Broken Link Checker: to search for internal and external broken links
MailChimp: to build your email marketing lists.
Ninja Forms: to easily create professional forms, for example, website contact forms, etc. 
WooCommerce: to create an eCommerce plan.