The drive-by pitch of Edddy is "3D Tiled".
Why mention it now? It recently became available as an AUR package!
And a freeze-bug, which too many ran into, was resolved.

Most of you are likely familiar with my elder brother's 2D FLOSS tile-based map editor called Tiled, which he began working on over two decades ago in QBasic. Since I could only find proprietary stand-alone tools, libre plug-ins/add-ons and voxel editors by the time I needed a 3D equivalent, I started Edddy. It's still functionally lacking and the formats are likely to change, but it can certainly be used to create levels where blocks only need rotating along the up-axis on a square prism grid. In fact levels for Blip 'n Blup, OG Tatt, A-Mazing Urho, KO and Tux! were already made using Edddy. The brush tool can draw 3D Bresenham lines and the fill tool works in planes, which both can save a lot of time. Crucial functionality that's still missing are selecting, copy-pasting and cluster painting... but undo/redo works. Of course it would also be nice - literally the primary issue - if it could convert models to the correct format on the fly, instead of expecting people to use the same engine.

If you think Edddy can be of assistance but need some help getting started or would like to contribute to its development, feel free to come aboard The Colourship.