This JAVA applet simulates the dynamics of the BTW sandpile model. It is paused now. To start running in a continuous mode push "Continue" button. To advance by one time step push "Next step". The heights of sand columns are color coded as:

	stable: green-3,blue-2, red-1,gray-0.
	active: shades of yellow and orange (see the colors at the row of checkboxes  at the bottom of the applet).

Boundary conditions along X and Y axes can be changed from open to periodic on the fly. Just push the button next to Lx or Ly textboxes. Changing values of Lx and Ly in these textboxes and pushing "Set new Lx, Ly" button starts the system in the new configuration with z(x,y)=3 everywhere except for one active site z=4 in the center. I set the maximal size of the system not to exceed 100. If you feel that your computer speed and display resolution allow you to simulate bigger lattices just download this page and Sandpile.class and set Lx, and Ly parameters defined by <param name="Lx" value="50"> <param name="Ly" value="50"> inside <applet ... ><\applet> block of this page to whatever values you desire. Then point your browser to the modified local version of this page. Good luck! 

You can add a grain of sand to the sandpile by clicking the canvas at the desired site. If you wish to paint on the canvas check the box corresponding to the desired value of z from the row at the bottom of the applet. Then clicking the canvas sets the site at the desired value of z. Clicking the "Fill" button fills the lattice with the selected value of z. To change back to grain-by-grain addition check "+1" box from the row at the bottom of the applet.

Finally, I set the delay between two subsequent time steps to 100 microseconds. You can set this variable in the corresponding textbox to a desired value and then click "Set new delay" button. Beware of setting this value to 0. When I was experimenting with this applet at my UNIX machine I noticed that setting small delay causes the applet to run very slow and browser itself hardly responds to mouse clicks :-(   May be one can write applet to avoid this problem but I don't know how to do it. However, on my PC setting delay to 0 works like a charm :-)