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Recommendations (for and against) are my personal opinion only and may be out of date.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
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.