Demo Bar: this plugin lets you display different theme demos for your audience to try.
Developer: this plugin optimizes your development environment, making sure you have all the tools you need to get to work.
Query Wrangler: lets you create queries as widgets or use shortcodes to include them on your pages.
Easy Query: this plugin lets you build custom queries easily. Lets you adjust query parameters and customize the plugin’s template.
Query Monitor: this plugin helps you see database queries performed on any given page and catch errors and problems.
Generate WP: comes with various tools to make life easier for developers. You have post type, sidebar, menu, widgets, and other generators to choose from.
FakerPress: generates dummy content to test your themes and plugins.
WP Ultimate Demo: this plugin lets you create a demo website to let your customers try your themes and plugins.
WP Example Content: it adds and removes example posts, allowing you to address design issues.
Plugin Demo: this plugin is developed for plugin coders. It lets your visitors try your script before making a purchase.
Bug Library: lets you add a bug tracking system to your website to catch and fix problems with your products.
LogPress: lets you log PHP errors, SQL queries, deprecated functions, login attempts, and everything in between.
WP Migration plugins: these plugins let you move your site from a local machine to a production server or one domain to another.
Code Snippets: lets you add code snippets to your website easily without modifying your functions file. You get a graphical interface to manage code snippets.
Theme Check: this plugin tests your theme against the latest standards and practices.
Performance monitors: these plugins let you monitor your plugin’s performance and catch potential issues.
Have you found better plugins for WordPress designers and developers? Please share them here.