Annotated Notes: For admin notes with automatic timestamps.

Control Panel Body Classes: Add body classes to various Control Panel screens for easier styling (in combination with Control Panel CSS).

Control Panel CSS: To make style tweaks to the control panel. See web/cp.css for styles.

Control Panel Nav: Custom navigation menus for each user type, for easier navigation.

Entry Instructions: Static instruction fields for the CP.

Environment Label: Shows which environment you're in, to help authors only enter content in staging.

Expanded Singles: Remove "Singles" grouping from Entries Index sidebar.

Field Labels: I consider this developer experience, since my alternative is having more fields with specific labels/instructions, but including it just in case.

Help Links: Create a dashboard widget with custom links.

Linkit: Makes it easier for authors to add different types of links.

Readability (on probation): Text statistics to help write more readable text (haven't really tried yet).

Sidebar entry types: Add entry types under their corresponding sections in the Entries Index sidebar.

Tags: See and manage all tags in one place.

User Manual: Build out a custom user manual right in the site, using a "User Manual" structure and entries. See also: https://plugins.craftcms.com/guide