Ah yes, 3d-construction kit, there was even a Sinclair-Spectrum version of that program.

First there was cube, and now there is Sauerbraten. Most likely the only two FPS-games with a realtime level-editor built in.

And it is open source too.
http://sourceforge.net/projects/sauerbraten/

Its actually dead easy to add your own textures, and the editor itself seems a bit odd at first since you only have one thing, TEH CUBE to play with. But take it from me, it is easy.

The status of this software at this moment is still pre-alpha, but if this is alpha?

You could also check out its older brother.
http://sourceforge.net/projects/Cube/

http://www.cubeengine.com and http://www.sauerbraten.org seems to have some problems at the moment, but i guess that they will be online again at some later time.

(there are some artifacts in the screenshot, but these do not show in the game itself, resized too :) )

Robert van der Veeke Basugasubasubasu Basugasubakuhaku Gasubakuhakuhaku!! "Better is the enemy of good enough." Dr. Mikoyan of the Mikoyan Gurevich Design Bureau.