This is a curated list of interesting tools used to author content for Quake. If you know of tools that should be listed here please leave a comment.
A conversion utilility for quickly turning 2D .BMPs into 3D Quake .MAPs
http://www.siliconslick.com/bmp2map/
FraQuake (Fractal Quake) is a tool for building fractal .map files.
https://www.quaddicted.com/files/idgames2/utils/level_edit/editors/fraquake.zip
Phase II makes Quake maps from rogue-like text files! It's an old 16-bit program and will likely need to be ran in a VM.