Voxel plugin - Very cool and promising voxel plugin. Has a free and paid version.
VFX
FluidNinja LIVE - Very detailed and modular plugin to use advanced dynamic particle systems. It’s paid but on the Discord there is an access to a free student version.
Utilities
Component Pointer Fixer - Helper to fix up broken C++ components added to blueprint classes.
Dependency Analyser - Useful to check dependencies count and size in your project.
Game Rig Tools addon - The main difference with the above addon is that you can CREATE and setup a new skeleton based on the UE4/5 skeleton. This addon is compatible with the above one to. Here is a good video that talk about how it works and how to use it.