Please see the below list for of helpful WordPress plugins. All of these are free and available from the WordPress plugin repository. To see the full list of WordPress plugins, click here.
This allows you to add conditions on when to show widgets (sidebar, footer, etc.). For example, you may add a text widget "Test Widget", that you want to show only on category pages, but not on the product page or other post types.
This is a breadcrumb plugin that allows you to show the breadcrumb trail throughout your WordPress site.
When your site is under construction or you change images in your WordPress site and media library a lot, this is a useful plugin that regenerates all the required image sizes to be used throughout your site.
Google XML Sitemaps
A simple sitemap generator for Google Sitemaps/Webmaster Tools that helps get indexed in search engines.
Custom Fields for Gutenberg
With the WordPress 5.0 they upgraded their classic editor to use the "Gutenberg" editor. Which by default hides custom fields. While you are building your content is helpful to see all of the custom field information. This plugin allows you to access those fields.
All in One SEO Pack
Should use one or the other (not together). For the importance of SEO and organic search engine ranking, these are the 2 most popular plugins to index your site in search engines, etc. Both pretty much do the same job just a matter of your preference.
GD Rating System
This is a star rating system that allows you to add/use star ratings and enable comments/reviews to posts/pages/products/upsells etc.
Contact Form 7
For creating and managing contact forms throughout your WordPress site, this plugin will let you build and administer these forms.
Import posts, pages, comments, custom fields, categories, tags and more from a WordPress export file.
Jetpack by WordPress.com
The ideal plugin for stats, related posts, search engine optimization, social sharing, protection, backups, security, and more.
WP Super Cache
A very fast caching engine for WordPress that produces static html files. This plugin generates static html files from your dynamic WordPress blog. After a html file is generated your webserver will serve that file instead of processing the comparatively heavier and more expensive WordPress PHP scripts.
For WordPress database/optimization/etc.
Advanced WordPress Reset
Reset and restore your WordPress database back to its first original status, just like if you make a fresh installation.
Advanced Database Cleaner
Clean database by deleting orphaned data such as 'revisions', 'transients', optimize database and more.