3D-XplorMath
Created by the 3D-XplorMath Consortium, an international group of mathematicians, 3D-XplorMath software is a free tool for visualizing mathematical objects and processes. Objects displayed include plane curves, space curves, surfaces, conformal maps, polyhedra, ordinary and partial differential equations, and fractals. The objects can be rotated and the parameters varied." (source: Intute)
The older, original version, runs only on MACs, but a newer JAVA version is also available.
The 3D-XplorMath website is hosted by the Department of Mathematics, University of California at Irvine.