What is ChaosEdit?
ChaosEdit is a Wolfenstein 3D modding tool in beta that was developed by Moritz Kroll (aka Ripper).
The program is capable of editing most aspects of the game, and has been actively used by the community for years despite lack of continued development or support.
How do I use ChaosEdit?
Can I change the window size?
Unfortunately, no. ChaosEdit is confined to a fixed window size.
Is ChaosEdit open source?
Ripper leaving the community was very sudden, and the chance of the source code for ChaosEdit ever being released is slim to nil.
Why are my walls all bright green in 3DEdit?
This happens because you have not loaded your VSWAP file in ChaosEdit, and may have only loaded your maps.
Solution
To fix this, either use the 
Load All button and choose your mod's folder, or 
Load Textures button and choose the VSWAP of the game you are editing.
Why aren't objects and enemies showing up in 3DEdit?
As with above, you may have only loaded your maps, and not the sprites from your VSWAP file.
Solution
To fix this, either use the Load All button and choose your mod's folder, or 
Load Sprites button and choose the VSWAP of the game you are editing.