Devel Feb 23rd, 2013 (updated Mar 30th, 2018) Essential module for developers. Should be disabled in production environments. Read more about Devel
Security Kit Oct 2nd, 2014 (updated Mar 28th, 2018) "Module provides Drupal installation with various security hardening options. This lets your mitigate the risks of exploitation of different web application vulnerabilities." Read more about Security Kit
Security Review Oct 2nd, 2014 (updated Mar 28th, 2018) "The Security Review module automates testing for many of the easy-to-make mistakes that render your site insecure." Read more about Security Review
Features Feb 24th, 2013 (updated Jul 7th, 2016) The Features module allows you to export configuration to code in an easy way. Read more about Features
Configuration Update Manager Jun 23rd, 2016 (updated Jul 3rd, 2016) This module is essential for configuration management in Drupal 8 Read more about Configuration Update Manager
Search Kint Jun 7th, 2016 Search Kint, introduced for Drupal 8, lets you search through Kint's (Devel's) debugging output. Read more about Search Kint
Strongarm Feb 24th, 2013 (updated Sep 14th, 2014) Makes site variables exportable. Uses Ctools. Usually used in conjunction with the Features module. Read more about Strongarm
Diff Feb 24th, 2013 (updated Apr 13th, 2014) This module integrates with so many other modules but out of the box can show you detailed node revision diffs. Read more about Diff