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Recommendations (for and against) are my personal opinion only and may be out of date.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
Unique field ajax (unique_field_ajax) 2017-02-08

d.o. page

"Essential"

Gives you a checkbox for 'Unique' when editing fields - can have it checked during form entry (via AJAX) and specify a custom error message per field.  Seems to have bug where field you're checking is sometimes refocused (even if no errors) if you've already tabbed away from it and continued typing.