DxWnd - Windows hooker - intercepts system calls to make fullscreen programs running in a window, to...
Unity - The multiplatform game creation tools for everyone.
Windowed Borderless Gaming - Play all your games in full screen windowed borderless mode.
Godot Engine - Feature-packed 2D and 3D open source game engine.
Simple Runtime Window Editor (SRWE) - Allows you to pick a running application and manipulate size, position, styles of its main/child windows.
Blender - Blender is the open source, cross platform suite of tools for 3D creation.