The Nebula Device is an open source 3D game and visualisation engine written by the now defunct Radon Labs .
It has support for scripting languages (e.g. Tcl, Lua, Python).
It is available for download at SourceForge. The binary distribution comes with customized interpreters with built-in 3d capabilities, and some demos.
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.