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Recommendations (for and against) are my personal opinion only and may be out of date.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
Conditional Fields (conditional_fields) 2021-08-10

d.o. page

Adds a 'Manage dependencies' tab to the admin page for content types.

This lets you set a 'target field' whose visibility is set according to a 'control field' and whether the latter is full/empty/checked/unchecked/has a specific value.

Another useful feature is you can prefill/pre-tick etc. the target field (including with a custom text value, for example).

To detect quickly if it's in use anywhere (I needed to do this before being sure I could uninstall the module on a site), export your site config and grep the /config/sync directory for 'conditional_fields' - it's stored in the 'third_party_settings' key in core.entity_form_display.node.[content type].default.yml