Zweck collection
The function f(x,y) = x2 y/(x4+ y2) is a shocking member of a function for which all radial limits exist and are zero but for which the limit along parabolas is nonzero. John Zweck from the university of Texas at Dallas has generated a nice collection of 3D printable models for multivariable calculus: You find Mathematica code here and illustrations. Here is a colored version of his visualization of the above function: