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Recommendations (for and against) are my personal opinion only and may be out of date.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
Address (address) 2018-11-24

d.o. page

Used to be called addressfield.  Adds one dependency - commerceguys/addressing.

This is a bundle of address fields, the correct combinations are shown depending on which country the user selects via a dropdown.
You can configure which countries are allowed per field instance.
To set the default, use the Manage Form Display tab.

It validates postcodes are in the correct format. 

Also supports 'zones' (groups of countries/subdivisions) for shipping/tax purposes.