AR Foundation allows you to work with augmented reality platforms in a multi-platform way within Unity.
Aero from Adobe is an app for viewing, building, and sharing immersive and interactive AR experiences.
Reality Composer by Apple is an app for producing and prototyping AR experiences directly in AR without prior 3D experience.
Oculus Quill is a VR illustration and animation tool that allows users to paint in 3D layers.
Tilt Brush by Google is a simple, easily augmentable tool for 3D painting in cross-platform VR.