Here's the source to a slightly complicated version of a Menger Sponge (see http://en.wikipedia.org/wiki/Menger_sponge ) renderer:

/***************
***BEGIN CODE***
***************/

boolean drawing;
float xr = 0;
float yr = random(0,TWO_PI);
color cubeColor;
int cubeCount;
int stages=2;
int pstages;

void setup()
{
 size(640, 480, P3D);
 noStroke();
 colorMode(HSB,1.0);
 cubeColor = color(random(0.0,1.0), 1.0, 1.0);
 drawing = true;
 pstages = stages;
 stages = 0;
}

void draw()
{
 if(drawing)
 {
   background(0);
   cubeMain();
   if(!mousePressed)
     if(stages<pstages)
       stages++;
     else
       drawing = false;
 }
 else if(stages != pstages)
 {
   drawing = true;
 }
}

void keyPressed()
{
 switch(keyCode)
 {
   case UP:
     pstages++;
     break;
   case DOWN:
     if(pstages>0)
       pstages--;
     break;
   case RIGHT:
     cubeColor = color(hue(cubeColor)+0.01, saturation(cubeColor), brightness(cubeColor));
     stages = 0;
     break;
   case LEFT:
     cubeColor = color(hue(cubeColor)-0.01, saturation(cubeColor), brightness(cubeColor));
     stages = 0;
     break;
 }
}

void mouseDragged()
{
 float rate = 0.01;
 yr += (pmouseX-mouseX) * rate;
 xr += (mouseY-pmouseY) * rate;
 stages = 0;
 drawing = true;
}

void cubeMain()
{
 cubeCount=0;
 lights();
 translate(width/2, height/2, 0);
 rotateX(xr);
 rotateY(yr);
 cube(0, 0, 0, 200, stages);
}

void cubes(float x, float y, float z, float s, int n)
{
 float u = s/3;
 for(float i=-u; i<=u; i+=u)
 {
   for(float j=-u; j<=u; j+=u)
   {
     for(float k=-u; k<=u; k+=u)
     {
       if(abs(i) + abs(j) + abs(k) >= 2*u)
       {
         cube(x+i, y+j, z+k, u, n);
       }
     }
   }
 }
}

void cube(float x, float y, float z, float s, int n)
{
 if(n>0)
 {
   n--;
   cubes(x, y, z, s, n);
 }
 else
 {
   pushMatrix();
   translate(x, y, z);
   cubeCount++;
   fill((hue(cubeColor) + cubeCount / (pow(20,stages) * 5)) % 1.0, saturation(cubeColor), brightness(cubeColor));
   box(s);
   popMatrix();
 }
}

/***************
****END CODE****
***************/

And here's the source for a simple enough example to be used in Learning, for example:

/***************
***BEGIN CODE***
***************/

boolean drawing;
float xr = 0;
float yr = random(0,TWO_PI);
int stages = 2;

void setup()
{
 size(200, 200, P3D);
 noStroke();
 fill(200);
 drawing = true;
}

void draw()
{
 if(drawing)
 {
   background(0);
   lights();
   translate(width/2, height/2, 0);
   rotateX(xr);
   rotateY(yr);
   cube(0, 0, 0, 100, stages);
   drawing = false;
 }
}

void mouseDragged()
{
 float rate = 0.01;
 yr += (pmouseX-mouseX) * rate;
 xr += (mouseY-pmouseY) * rate;
 drawing = true;
}

void cube(float x, float y, float z, float s, int n)
{
 if(n>0)
 {
   n--;
   float u = s/3;
   for(float i=-u; i<=u; i+=u)
     for(float j=-u; j<=u; j+=u)
       for(float k=-u; k<=u; k+=u)
         if(abs(i) + abs(j) + abs(k) >= 2*u)
           cube(x+i, y+j, z+k, u, n);
 }
 else
 {
   pushMatrix();
   translate(x, y, z);
   box(s);
   popMatrix();
 }
}

/***************
****END CODE****
***************/

I hope you love this fractal as much as I do.

-- Lukstr