Unity - The multiplatform game creation tools for everyone.
placeholder.pics - Lightest way to include placeholder images in your design
Godot Engine - Feature-packed 2D and 3D open source game engine.
PlaceKitten - Dead-easy placeholder images. Of kittens.
Blender - Blender is the open source, cross platform suite of tools for 3D creation.
Fill Murray - Custom-sized placeholder images of Bill Murray