Constraint-based three-dimensional dynamic geometry
From a thorough web search, there does not seem to be a dynamic geometry software package which (a) began its life handling three dimensions, rather than just two, and (b) allows you to express the desired geometric configuration in terms of constraints on the entities (e.g. l and k are parallel, a, b, and c a collinear, etc.) rather than as a construction (e.g. l is the perpendicular bisector of a and b). The goal of the dyna3 project is to close this gap.
Note that currently this is just the barest beginnings of the project, more of a framework for developing dyna3 rather than anything useful.