This list is kept short and only updated based on plugins actively used on live sites or thoroughly tested locally.
For Development Use
Developer, a plugin to help WordPress developers work in a local environment. It has various complementary tool for debugging, translation, right to left reading, accessibility testing etc.
Query Monitor by John Blackbourn is an excellent complement to developer tools. It filters database queries and provides with an extensive report, sorted by query type, function etc. For example, it can help you finding slow queries or conflicting plugins.
Theme Check scans a theme and runs test based on WordPress.org Theme Review Team. It gives you hints about what is required to comply to WordPress.org standards.
(WordPress Beta Tester)
https://wordpress.org/plugins/wordpress-beta-tester/ by Peter Westwood Once enabled this plugin switches your site onto the latest WordPress development version.
WordPress lets you update to the latest nightly build automatically, so you can test the latest version in development without this plugin.
For live sites use
Regenerate Thumbnails, by Alex Mills regenerates the thumbnails for all your image attachments.This is useful for instance when you change the thumbnails sizes in your templates, i.e. if you use a custom function like
WordFence Security is a 100% free and open-source security software supported by a large team dedicated exclusively to WordPress security.
WooCommerce is a popular plugin for small sites and indie developers. It is now part of Automattic after its 2015 acquisition. The basic plugin is free, with more options available as paid features. Theme integration is not that complicated but time consuming. Granted, e-commerce on a small self hosted WordPress site can be a bit of a headache.
I only use it for one of its multiple features but it deserves a mention for posting in Markdown alone. Jetpack by WordPress.com
Sometimes, good things come to a stop. Here are some plugins that were promising but are no longer supported or were abandoned by their authors.
Sadly, Scroll Kit development was discontinued after its acquisition by Automattic in April 2014. ↩