This is an attempt to collect sources of demos, tools and utilities related to Quake. The stuff on these pages has been written by others, and is copyrighted by the authors.
John Carmack has released the Objective C source of the NextStep QuakeEd editor, which is avialable here for download. Map files are available as well. You will find a Source overview page and some GUI screenshots in the QuakeEd section.
The source of the DOS release of Brian Martin's MedDLe viewer has been released, including a 3DS to MDL converter. The mdlv1src.zip archive includes a binary as well.
The example clipping source ddjclip.zip archive has been written and published by Michael Abrash, and is described in an accompanying article.
The source in the ddjzsort.zip archive has been written and published by Michael Abrash, and is described in two articles.
The four implementations of perspective texture mappers, one of them with pipeling FDIV assembler, are collected in the texture.zip archive. They have been written and published by Chris Hecker in a recommended article series.