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Recommendations (for and against) are my personal opinion only and may be out of date.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
Flag (flag) 2017-03-13

d.o. page

"Essential"

Handle favourites, reading lists, spam, friend lists etc.  Can be applied to any entity (nodes, users, comments etc.)  It appears as a clickable link. Can choose between page reload and AJAX when user toggles it.  Linked with views, and you can trigger events when flag count reaches a threshold.