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Recommendations (for and against) are my personal opinion only and may be out of date.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? I will be adding this, but as of August 2021, this site you are reading is the only Drupal site of mine I've been able to upgrade to D9, so although you can check D9 compatibility status of all modules (use upgrade_status, it's superb) I can't speak from personal experience about any of them.

(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.