JannisX11/blockbench
This is a surprisingly well-featured low poly 3D model editor that is open source and pretty easy to use. I have had a lot of fun recently using this to make silly things to add to my sons’ Minecraft games.
Considering I had no experience with creating assets for video games I was still able to quickly get some things working with this which was very cool and a great opportunity to show my kids what I do (software engineering) in a way that was tangible for six year olds.