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Recommendations (for and against) are my personal opinion only and may be out of date.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 February 2022 I've updated all 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.

What about sites still running Drupal 7? In short I still recommend leaving them on Drupal 7. Your decision is really if you will gain anything from Drupal 8/9 and for many then answer will be no, and one of the main hassles - apart from converting each content type one at a time, and setting up a new theme, will be recreating all your views by hand, as views can't be automatically upgraded.