Since WordPress 3.7 updates, most of the WordPress installations are able to run and apply all new updates in background. Background Update Tester is the new WordPress plugin that tests your WordPress site for compatibility issues and explains if problems are found.
JB Common is the new WordPress plugin that enables some basic common features that are not packed with default WordPress install like adding favicon, meta tags, adding HTML to head and footer, WordPress e-mail from header filter and a few more.
WordPress has a new debug plugin that allows super admins to access debugging information from WP admin bar.
Monkeyman Rewrite Analyzer understand your rewrite rules (“Permalinks”). It is indispensable if you are adding or modifying rules and want to understand how they work (or why they don’t work).
A quick and easy method for showing all core widgets of WordPress in a sidebar for testing purpose. Automattic’s Monster Widget plugin does this very easily.