This is based on the plot Z=X*Y (known as a hyperbolic paraboloid or a saddle point) but instead of being a continuous surface, I've discretized it so that it is formed from a collection of varying height square prisms.
Aside from looking great, it feels really nice to run your fingers over the discretized surface.
Three STL files included:
hyperbolicParaboloid.stl is for a machine with a single extruder. it is formed from 16x16 square prisms, where each square is 3mm per side.
hyperbolicParaboloidOddSquares.stl and hyperbolicParaboloidEvenSquares.stl are used for a dual extruder machine so that every other square is a different color (same dimensions as above)
Finally I've include the openSCAD source which allows you to parametrize the number of squares per side as well as the size of each square.