Surface Studio

Its a tool for unity editor which allows you to customize your texture directly inside unity.

You can just paint over your texture in 3D view and apply to model with ease.

Contains:

Mirror mode for X/Y/Z axis.

Custom decals/brushes.

2D/3D view with split mode.

Painting over few assets at once.

Support for shaders to include presets for exporting.

and more...

To open it in unity top bar you should have "Tools" section and there "Surface Studio -> "Open Studio"