We start the software-of-the-week series with Jenn3D. I don’t know what Jenn stands for, but my children say it has to do with “genial”. It’s a free program for Win and Mac; sources are available and can be compiled for other platforms. Look it up at http://www.math.cmu.edu/~fho/jenn/#download
The program presents a wide variety of regular and semiregular polytopes, that get to be embedded into the 3-sphere and stereographically projected on the affine 3-space. The resulting 3D object can be “ordinarily” manipulated inside the screen. To fully understand the program, it’s best to have an idea of conformal mapping in 2D and 3D, and an intuitive notion (if not a formal one) of projective plane and projective 3-space, but it’s really enjoyable by anybody, even supposed math-haters. Yes, it was tested by our team under the severest conditions.