These plugins do the amazing and make working with WordPress so much more powerful, and flexible. Most are free, some are paid. All are worth your time and donations for the plugin creators.
All-in-One WP Migration by ServMask – if you miss using the Blog Vault migration when importing to WPEngine this is your savior. Free for small sites. Works like a champ at migrating WordPress sites with a simple interface.
Better Search and Replace By Delicious Brains – A small plugin for running a search/replace on your WordPress database. A must-have plugin for migration clean-up, unless you like writing DB queries to use in phpMyAdmin.
Duplicate Page by mndpsingh287 – Duplicate Posts, Pages and Custom Posts using single click. Note that the Divi theme now has this functionality built in.
Gravity Forms – The easiest to learn, most extensible, and reliable forms plugin for WordPress.
GravityView by GravityView – The best, easiest way to display Gravity Forms entries on your website.
Members by – Justin Tadlock – A user and role management plugin that puts you in full control of your site’s permissions. This plugin allows you to edit your roles and their capabilities, clone existing roles, assign multiple roles per user, block post content, or even make your site completely private.
Stream by XWP – Stream tracks logged-in user activity so you can monitor every change made on your WordPress site in beautifully organized detail. All activity is organized by context, action and IP address for easy filtering.