KenShape is a tool to create 3D models from 2D pixel art which is really clever, while Asset Forge allows you to create 3D models from various parts. Both aimed at game developers not normal users but still fun to play around with, created by Kenney who is well known for producing tons of high-quality public domain art assets.
Recently Asset Forge had a version 2.2 release which added colour map (UV mapping) export option, along with 33 new blocks to use for the standard version and a further 21 if you have the Deluxe edition along with a number of bug fixes.
The other application is KenShape, which had the 1.3 release recently. Along with updating the version of Unity used it also added a 64×64 project size, depth generation based on luminosity, depth generation based on colour palette index, depth generation based on noise, scaling option when exporting a model, additions to the GIF and PNG exporters and more. It's shaping up to be quite a powerful tool.
I tried to make a Floppy Disk icon in it, if that wasn't clear…really interesting software though, even for someone who is not an artist or particularly good at making models, it can be a wonderful stopgap. In fact, both the applications mentioned are great for prototyping.