The Nebula Device is an open source 3D game and visualisation engine written by the now defunct Radon Labs .
Try it out... it's impressive.
Nebula Device uses a special version of Tcl (MicroTcl). It is reduced to 36 commands for 160 Kbytes.
dbohdan 2014-12-26: The the original creator of Nebula Device has launched a new experimental cross-platform C++ game engine called Oryol . It has no support for Tcl or other scripting languages yet AFAICT.
m-nebula - medical graphic engine (also still be a game engine) used in therapeutic systems. Modification of Nebula1 game engine, originated by Radon Labs GmbH and continued and modified by InSightec Inc. Script support: Tcl and Python.