SuperSplat is a free and open source tool for inspecting and editing 3D Gaussian Splats. It is built on web technologies and runs in the browser, so there's nothing to download or install.
BRL-CAD is a powerful cross-platform constructive solid geometry solid modeling system that includes an interactive geometry editor, ray-tracing for rendering & geometric analyses, network distributed framebuffer support, image & signal-processing tools.
Tartan is an OpenGL texture manager library written in C/C++.
It handles loading textures from file, for example from tga and pcx, and making them available to OpenGL.