These are WordPress plugins (mostly free) that I personally use and recommend. They are not required for your theme to function, unless specified in the theme demo.
If the standard Post or Page layouts aren’t doing it for you, you need Advanced Custom Fields.
Both of these plugins are good choices for your WordPress SEO efforts.
Add a contact form to your WordPress site, or any other similar form. Customize your own fields and emails.
Automatically block spam comments.
Enable Accelerated Mobile Pages (AMP) on your WordPress site. Google adopted the Accelerated Mobile Pages project and added it to mobile search results in February-March 2016. AMP is designed to make web pages faster to load for users by slimming down elements users need to download when visiting a web page.
Get a visual editor for your widget text boxes.
Toss up a “coming soon” page while you’re developing or updating your site.
Increase page views by suggesting related posts at the end of your blog articles.
Easily add all 600+ Google Fonts to your website with this plugin. If you have text in widgets, make sure to click the “Automatically add paragraphs to text” checkbox so that the fonts can be applied to the <p> tag around those widgets.
Gather donations for your organization. Great plugin for the Do Good 501(c) theme.
This is a great alternative to MailChimp with tons of free functionality, and it integrates with WordPress and WooCommerce.
OptinMonster is what powers those pop-up sign-up boxes on this website and many others. It has a WordPress plugin for integration into its system.
Add all those social media sharing buttons to your site.
If your theme didn’t come with a box to add your own CSS, you can add one with this plugin. This is a good way to edit your site’s CSS without editing the main stylesheet, and therefore losing your edits with an update.
Generate a table of contents from the headers on your page. See it in action on the documentation pages on this site.
Generate a list of the top 10 most trafficked pages or posts.
Get automatic backups of your site. Lots of options for where to save the backup files.
The industry standard method of adding ecommerce functionality to a WordPress site.
Import and export your WordPress content and move it to another WordPress website.
Compress and optimize your images to increase page load times.
Install this to speed up site performance. Don’t enable it until after you’re done with development.