Some quick hacks - not polished - hardly usable - use at own risk. The source code is available, too.

An implementation of the command I keep typing all day long.

For most of those, you'll need a browser with support for the <canvas> element (that is, any modern browser should work). If in doubt use the latest Firefox or Chromium.

My entry for the 26th Ludum Dare 48h game programming competition.
Moore's Game
My entry for the 24th Ludum Dare 48h game programming competition.
A self-contained single-file minimalistic HTML5 slide-deck.
Voronoi Mandelbrot
An interactive, zoom-able real-time rendering of the Mandelbrot set, using random points and Voronoi tessellation.
A pixel-based grey-scale Mandelbrot set renderer with super-sampling. It renders in arbitrary resolutions and works pretty well for generating desktop wallpapers.
A line-based image-to-ascii converter. It doesn't work too well, but it's so simple it's a wonder it works at all. Please use Chrome or Chromium for this one.

Some quick hacks written in the visualization language Processing. These are Java applets, so you'll have to have a JRE installed on your machine. (Sadly, you'll even need the Sun^H^H^HOracle JRE, others won't work.)

Canvas Game of Life
A living canvas.
Fractal Garden
A very simple fractal viewer.
Fractal Garden 2
More or less a combination of the two above.
The Dark Side
An interactive version of Pink Floyd's famous Dark Side of the Moon album cover.

The quickest hacks of them all. And the most useless. You'll need a POSIX system.

mod-time graph
Plots a graph of the modification times of all the files in ~/src.
Plots a graph of the ping responses of the hosts with a two-character DNS name.