You may find these useful.

• Kenney Assets
• QuaterniusDev
• AmbientCg

Unreal-only:

• The Epic Games created content: https://www.unrealengine.com/marketplace/en-US/profile/Epic+Games
• The Permanently Free collection: https://www.unrealengine.com/marketplace/en-US/assets?tag=4906
  

We are also blessed to have amazing artists from our own community release high quality artwork under public domain:

• AmberFallStudio

I will add more as I find them. Feel free to also post replies with more resources, but if they are not public domain, please mention what license they use.

Here you go, I'll share my personal list of free ressources and assets :

Textures:

• textures.com
• 3DXO
• 3D Textures
• Texturer
• 3D Total

Different types of ressources / assets:

• Quixel Megascans (textures and assets)
• Free 3D (models)
• PolyHaven (game assets)
• World Machine (terrain generation)
• Glitch game art (CC0)

Thanks for AmbientCg, I didn't know this one. Even though Quixel was bought quite recently by Epic Games, their assets are still usable with Unity.

You will find that megascans cannot be used anywhere except unreal without a paid plan

www.opengameart.org

I thougth ytihs might be usefull

here for sound effect 

and music

https://pixabay.com/

Regarding the use of Megascans with Unreal Engine, it seems the guys at Quixel are not fond of open source projects using and redistributing their assets: https://help.quixel.com/hc/en-us/community/posts/360008583998-Megascan-textures-in-open-source-Unreal-projects-

Would it be acceptable to submit the project source without the Megascans assets themselves but a list of their ids so other UE users can download them on their own then?

Thanks for all of these amazing resources!

Launched literally yesterday: THE BASE MESH

Lots of royalty-free music at freemusicarchive.org! There are varying licenses depending on the artist and even song, so make sure you look at that before using. Lot of tracks with only attribution/credit required, even for commercial projects.

This is a direct link to a search on there for strictly public domain works! https://freemusicarchive.org/search?adv=1&music-filter-public-domain=1&music-fil...

edit: Downvoted for sharing a site with free public domain music?.. o.O

are mixamo animations allowed?
edit: on their faq it says animations are allowed on royalty free projects 

https://helpx.adobe.com/creative-cloud/faq/mixamo-faq.html