Useful Drupal modules

Click on the module name for notes, comments, patches.
Recommendations (for and against) are my personal opinion only and may be out of date. Feel free to email with corrections/suggestions.

Why haven't you mentioned Drupal 9? As of December 2021 I've updated most of the sites I look after to Drupal 9 (experience: mixed). I haven't decided how to label future version compatibility as D10 will be coming soon as well. The upgrade_status module is superb. You're bound to have issues with composer. Also watch out for a nasty session headers bug with redirect_after_login.

(1 modules in list)
Module name or machine name
Any text in the notes
Name D8 Personally Tested Last Updated
Warden (warden) 2018-09-17

d.o. page

A fork of the System Status module, but with a Symfony app for you to install to act as a server, rather than using a paid third-party service.  Monitor core/module update status of multiple Drupal sites.  If you're familiar with Nagios and NRPE, it's a little bit like that, but for Drupal modules, and more lightweight.  Note the server uses MongoDB, so you need to install that, plus the PHP extension, and it communicates with a public key-pair, though it should set that up itself.