Calculus you can see: build it, sweep it, turn it around.
Nielson Widget Studio is six math labs. Build a figure (points, vectors, curves, regions, planes, solids with their shadows, quadric surfaces) and draw it in one part at a time. Type an integral and watch the area sweep in. Spawn vectors, points, lines, planes and fields into one scene and type how they relate (u + v, u × v, proj(u, v)); find where lines and planes meet; ride a path through a field adding up work and flux, place curl meters, and check Green's theorem. See one point in rectangular, cylindrical and spherical coordinates, and why dV picks up r or ρ² sin φ. Read a surface through its slices, tangent plane, gradient, critical points and Lagrange points. Watch a double or triple integral fill its region slice by slice, in any order, in Cartesian, polar, cylindrical or spherical.