Here is a little Diffusion Limited Aggregation program. 
What it does:
  1.  seed the bottom row with dots
  2. then make a dot randomly placed above everything
  3. let the dot do a little brownian random "dance"
  4. if it ends up next to any other dots, make it stick there
  5. go to step 2 unless above the maximum height of the applet
Here is the source if you feel like reading it.