Starting back in November 2018, Epic Games started giving away monthly Unreal Assets, which continues today. Most of the content is free but must be “purchased”, at which point it is yours forever. Some of the assets will remain free forever, no just for this month.
February free assets:
Hand Painted Texture Starter Kit
Luos’s Particle Toolkit Vol 1
Platformer 2D Toolkit
Scifi Kitbash Level Builder
Free forever assets:
Particles and Wind Control System
You can learn more about the assets on the Unreal blog here.