This is an interactive Voronoi diagram (more information on Wolfram MathWorld and Wikipedia). A modified version of this code is also used in the header above the navigation buttons on the main pages of my website.
I think it is particularly interesting to see how using different metrics affects the result. I recommend adding a few points (click the canvas) before looking at the result of different metrics (press 'n').
In this demonstration, I have implemented 3 different metrics. The table below describes them. Here, I define the point \mathbf{a} = (x_1, y_1) as the point of interest (represented by black dots on the above display), and \mathbf{b} = (x_2, y_2) as any other point on the plane.
Name 
Measurement 

Manhattan distance 
\lVert \mathbf{b}  \mathbf{a} \rVert_1 = \left  x_2  x_1 \right  + \left  y_2  y_1 \right 

Euclidean distance 
\lVert \mathbf{b}  \mathbf{a} \rVert_2 = \sqrt{(x_2  x_1)^2 + (y_2  y_1)^2}

Chebyshev distance 
\lVert \mathbf{b}  \mathbf{a}\rVert_\infty = \max( \left  x_2  x_1 \right , \left  y_2  y_1 \right  )

Alternatively, you can press '1' to use the Manhattan distance, '2' to use the Euclidean distance, '0' to use the Chebyshev distance, 'n' to toggle through all metrics, and 'r' to clear the display.
This page was linked on Agile Scientific's blog post The norm: kings, crows and taxicabs by Matt Hall. Welcome, new visitors! :)
You can get the source code here (ZIP file, 20 kB). You will need the Processing software package to run the program.