Visualizing with Grapher

Here are a few online services which offer graphing functionality:

Desmos (2D only)
Math3d—a bit rough around the edges, but works well once you figure out the quirks. From what I can tell it doesn't understand juxtaposition multiplication with variables, i.e. you need to write $\sin(n\cdot\theta)$ instead of $\sin(n\theta).$

Graph a torus with major radius $K$ and minor radius $k.$
Answer

$\begin{bmatrix} (K+k\cos b)\cos a\\ (K+k\cos b)\sin a\\ k\sin b \end{bmatrix}, \quad a=0..2\pi, \quad b=0..2\pi$
Graph a Möbius strip.
$\begin{bmatrix} x\\y\\z \end{bmatrix} = \begin{bmatrix} \cos(\theta)(R - t\sin(n\theta/2))\\ \sin(\theta)(R-t\sin(n\theta/2))\\ t\cos(n\theta/2) \end{bmatrix}$