I've been working on an editor/modeller for distance field based models/scenes that can be rendered with a raymarcher/sphere tracer, for example in a fragment shader. The scene can be saved to and loaded from XML but more important is the generated scene source code for your (or my) raymarcher. Tired of modelling in code? Try Zwerg!
Zwerg is limited to only the very basics of SDF modelling and is missing many useful features any 3D editor should have. It's a helpful tool for a small group of programmers.
If you're an artist or are looking for a tool for SDF/CSG based modelling, I highly recommend MagicaCSG.
All function definitions are loaded from an external XML file and can be modified to your needs. Mercury SDF Library functions are integrated (most of it).
Feel free to fork the project on github.
Source Code (1.4.1)
Binary download (1.4.1)
(Windows only, requires .NET4.5)
WebGL Version (experimental)
Here you can see how Zwerg is used to create a raymarched scene for a fragment shader (SIGGRAPH 2015, Shadertoy competition)