Published on 13 Oct 2010.
There's a nice page about this type of game at answers.com:
The Falling Sand Games are a series of particle-based computer games which allow the user to place particles of different elements on a canvas. The particles can interact with other particles in various ways, and may be affected by gravity. Many complex effects may be achieved. For example, fire can be made to move upwards, burn elements, and emit smoke. Game physics files (mods) are generally standard ASCII text files which tell the game how to control the particles. Many versions of the Falling Sand Game have been written since its introduction in 2005.
Here's one to try out online.