MapMixer

Have you ever wanted to switch out just the THINGS or LINEDEFS for a level? No? Well now you can!   

MapMixer is a command-line tool that mixes map elements from a custom WAD into a base WAD and outputs the result as a WAD you can boot up in any Source Port!  

The tool is also capable of recognizing when non-map lumps are included, in which case they will replace the existing lumps in the WAD, or be added to the end if they do not already exist.  

The current version supports Doom I/II.

The source code is available here.

Obligatory Copyright Note: This tool is only meant to be used locally with your Doom IWADs, please don't go sharing around the patched WADs, as that is still unfortunately piracy. Please respect id's copyrights and whatnot. That being said, you are free to share the patch WADs however you want alongside basic instructions.