At the Berea College Museum, we are having success using Virtus Walkthrough, an
in expensive 2D, 3D visualization program for Macintosh (I don't know if there
is a PC equivalent).
 
It cost us only $49.00 and is simple enough that undergraduate students (not
math or computer majors) can use it right away.  It lets you draw in 2D to
scale, then view the space in 3D and even walk through it in real time.  You can
program a walk path and record it, even output a walkthrough to video tape (with
the right hardware).  You can print 3D views of a space from any point in the
space, even on a cheap laser printer.
 
Being visualization oriented, it will not let you output scale floor plans and
is not designed for a high level of accuracy.  I expect that anyone who has used
a high power CAD program might be frustrated by its limitations.  It is near
perfect for our situation, but perhaps not for everyone.  I use it only for
planning and testing ideas.  I don't know if it could be used to by designers.