I like to dabble in a range of different apps, including GIMP, Inkscape and Blender. I see a lot of tutorials showing how to do 3D-style effects in GIMP and its competitors, and I wonder: why bother? You can achieve much more convincing and elaborate results in a proper 3D app like Blender: by all means, start with some graphic created in GIMP or whatever, then bring it into Blender as an image texture, and apply effects to it there. Also it includes a range of procedural, non-destructive effects via its Shader Nodes and Geometry Nodes features. This means you can change the parameters of the effect and re-render, without making permanent changes to your original graphics, so there is no accumulation of quality loss.

Has anybody else been experimenting with combinations of open-source tools in this way?

Another thing I like about the tools I mentioned is something else they have in common: Python scriptability.

This is OK if you create the whole image from scratch. But very often with Gimp you are adding something that should fit the existing image "well enough" (in other words, you don't need perfection) and it would not be trivial to create the "mannequin" object in Blender from what you have in Gimp (angle of view, perspective, lighting (direction, color temperature), material...).

I often think about 3d graphics and make them all the time as fractals with Mandlebulber. (There are also 4d fractals. Hypothetically a 4D fractal is our universe.)

3d or 2d?
The difference is that 3d graphics have x,y,z dimensions while the Gimp workspace is x and y. In Gimp you can map an object to a sphere or a box and then rotate it. Perceptually it's 3d because you can look at all sides of it but not at the same time. People are looking at 3d objects and 2d objects on a flat screen or as a flat image so 3d is often a perceptual illusion. Unless you 3d print your object. All video and pictures are the same. They are representations of what is 3d.

I have used Blender and agree with vittforlinux. It's 3d modelling that can make a physical object as true 3d but how often is it used for that?

I used Gimp to make a path that I imported into blender to make a Chrome Chain: viewtopic.php?f=22&t=17523

It came out great. I rarely use blender and every time I get around to using it, I have to learn it all over again. It's a great program, but it's definitely not for the faint of heart. I'm all for Free Open Source Software and use them exclusively on all my projects.

I seem to remember watching an episode of Jay Leno's Garage, where the used gimp to manipulate an image of a part and save to .svg, then input it into a 3-D printer to make a custom part for an antique car. I thought that was cool!

Nothing wrong with thinking outside the box to get the job done, I always say.....