Tools of the trade

http://awesomebump.besaba.com/
Great new open-source texture creation tool (high quality normal maps from photo sources etc.)

The cake was a lie! Again! ... doesn't work....

Needs OGL 4.0 and doesn't seem to run with Optirun/Bumblebee on Linux, but otherwise it does work just fine.

Here's a potentially useful tool linked to me by TimePath;

https://github.com/a1studmuffin/SpaceshipGenerator/

Uses Blender to create procedural spaceships, possibly for use as map objects.
Enjoy!

Online tool to generate normal, bump and specular maps:
http://cpetry.github.io/NormalMap-Online/

fSpy (Windows, Linux, Mac - FOSS)

A tool for still image camera matching. You load an image and then define the vanishing points. The tool then calculates the image's perspective. You can load the project file into Blender, which will automatically create a correctly aligned camera with your picture as a background image. This is incredibly useful for modeling from reference images.

Krita (Windows, Linux, Mac - FOSS)

A powerful drawing program, very useful for texture work. In my opinion better than GIMP, because it has nondestructive filters.

Inkscape (Windows, Linux, Mac - FOSS)

A vector art program.

Materialize (Windows, Linux only with Wine - FOSS)

Description from the website: "Materialize is a stand alone tool for creating materials for use in games from images. You can create an entire material from a single image or import the textures you have and generate the textures you need."

There is no native Linux version, but you can run it in Lutris / Wine. You have to install the Arial font via Winetricks though or the UI doesn't have text.

Meshroom (Windows, Linux - FOSS)

A photogrammetry program that turns individual photos into high-res 3D models. Required an NVidia graphics card with CUDA. If you have the money, I suggest buying Agisoft Metashape, because it's much faster and creates better results.

Instant Meshes (Windows, Linux, Mac - FOSS)

Retopologizes high poly meshes into evenenly sized quads or triangles.