Doom Construction Kit

Doom Construction Kit is a sector-based DOS level editor created by Ben Morris. It is similar to DEU in many respects; for example, DCK is mode-based.

Versions of DCK prior to 2.2f were freeware; those released afterwards required a payment. However, older versions can still be downloaded and have the same features as later versions.

(The Waite Group published an unrelated book also called "The DOOM Construction Kit".)