"This module performs additional validation when a node is created or updated by a user to require that a node's title, author, language, and CCK fields are unique within a given context." There are many different contexts including content type. So, for example, you could make sure that nodes of a certain content type have unique titles.

Included in Drupal 8 core, this plugin gives a nice javascript walkthrough on any page.

Insert is the easy, secure way to let editors add images and links to files using WYSIWYG editors.

An example module which integrates the Flag module with a custom form.

Gives replacement tokens. This module is used by so many other modules, you are sure to use it and learn it if you haven't already.

"The workflow module allows the creation and assignment of arbitrary workflows to Drupal node types. Workflows are made up of workflow states. For example, a workflow with the states Draft, Review, and Published could be assigned to the Story node type."

" Token filter is a very simple module to make token values available as an input filter."

"This module defines the "hierarchical_select" form element, which is a greatly enhanced way for letting the user select items in a hierarchy."

"Define dependencies between fields based on their states and values. "

Use PHP to create computed fields. If you use this, export the configuration so you don't have PHP in the db!