RaiderRanked

Shows your actions per minute live on screen, and records every fight with an APM curve and a timeline of which spell you cast when.

CircleOnCursor it does what says

X-Perl Forever brings the classic X-Perl unit-frame experience to WoW Forever, with compatibility fixes for its modern interface and protected-value systems.

Hide Anything is a simple and lightweight Addon, you can basically hide anything in your UI and customize many features which Blizzard do not allow to be hidden.

Shadowed Unit Frames updated for Forever and Midnight.

Raid Bar is a useful tool that gives you faster access to marking and dropping world markers. A more appealing utility tool compared to the default blizzard UI.

Bringing back The Players ability to make "Meaningful choices" to track their teammates CDs.

Better Cast Bars is a lightweight cast bar overhaul that improves visibility and control over enemy and player casts. It provides clear, customizable cast bars for nameplates, player, target, focus, and enemies.

Standalone version of EllesmereUI Raid & Party Frames, a lightweight, high-performance replacement for Blizzard raid and party frames with powerful visual editing tools.

Cell_UnitFrames adds custom unit frames, resource indicators, icons, buffs, debuffs, and flexible UI options for WoW Classic. Works with Cell, supports profiles and localization.

A complete interface replacement for World of Warcraft: minimal, information-dense, and configured out of the box.

Click Binds for Plexus

A raid frame addon inspired by Cell. The Idea grew out of maintaining my own fork of Cell.

Debuff tracking, dispel and CC highlights, sound alerts, and quick-click actions — all in Decursive-style party frames. Pre-configured for all class dispels and talents, so it works out of the box. (Formerly SmartDebuff Reloaded)