Doom level editor
Yadex is a Doom level (wad) editor for Unix systems running X, including Linux. It supports Doom alpha, Doom beta, Doom, Ultimate Doom, Final Doom, Doom II, Heretic and also, in a more or less limited way, Hexen and Strife. Yadex is descended from DEU 5.21. Therefore, as you might expect, it's a rather low-level editor that requires you to take care of a lot of detail but on he flip side allows you to control very precisely what you are doing. In addition, it has many advanced functions that DEU didn't have, to make certain tedious tasks easy.