OpenSCAD is a free, open source modelmaking program that's easier to use than you might think.
3d model print parameters
This thing was designed in OpenSCAD, and you can follow the design process here: http://www.3dgeni.us/impossible-pyramid/ When you print it, it needs ...to be aligned so that the gap in one of the sides is flat on the bed. After printing, rotate the whole object before you photograph it.