Great!

I made a small change to have it as a screensaver. It randomly produces MouseClicks to keep the show running (see mouseClickedFAKE   ).
Changing color now, use 'n' to restart.
I couldn't let him do a nice change of radius though.

Going to a game of pool shouldn't be too hard.

Greetings, Chrisir



int numballs = 41, magnet = 0;
float friction = .999f;
// rad = 30
float maxSpeed = 15;
float clusterRad = 1.1f, clusterMult = 1.0f;//Play with these values to get grouping/clustering


Ball[] tableballs = new Ball[numballs];


void setup() {
 size(screen.width,screen.height);

 ellipseMode(CENTER);

 MySetup() ;

}

void MySetup() {

 float x = 0, n = 0;
 tableballs[0] = new Ball();

 tableballs[0].cx = 0.9f * width;// '0 is cueball
 tableballs[0].cy = height / 2;

 x = (int)(random(1) * 0.5f * width);

 for (int i=1; i<numballs; i++) {
   tableballs[i] = new Ball();

   tableballs[i].cx = x;
   tableballs[i].cy = (.5f * tableballs[0].cy) + (n * (tableballs[i].rad * 2)) + 0;
   n++;

   if (tableballs[i].cy > 500) {
     n = 0;
     x = x + tableballs[i].rad*1.3;
   }
 }
}

void keyPressed() {
 if (key == 'm') {
   magnet = 1;
 }
}

void keyReleased() {
 if (key == 'w') {
   clusterRad += 0.1f;
   println(clusterRad);
 }
 else if (key == 's') {
   clusterRad -= 0.1f;
   println(clusterRad);
 }
 else if (key == 'm') {
   magnet = 0;
 }
 else if (key == 'n') {
   MySetup() ;
 }
}


/*
void mouseClicked() {
float R = 0, ang = 0;

R = dist(mouseX, mouseY, tableballs[0].cx, tableballs[0].cy);

if (R > maxSpeed) {
R = maxSpeed;
}

ang = atan2((mouseY - tableballs[0].cy), (mouseX - tableballs[0].cx));

tableballs[0].vx = R * cos(ang);
tableballs[0].vy = R * sin(ang);

}
*/

void draw() {

 background(127);

 float ColDist = 0, ang = 0, R = 0, dx = 0, dy = 0;

 for (int i=0; i<numballs; i++) {
   tableballs[i].cx = tableballs[i].cx + tableballs[i].vx;
   tableballs[i].cy = tableballs[i].cy + tableballs[i].vy;
   tableballs[i].vx = tableballs[i].vx * friction;
   tableballs[i].vy = tableballs[i].vy * friction;

   if (tableballs[i].cx < tableballs[i].rad) {
     tableballs[i].vx = -tableballs[i].vx;
     tableballs[i].cx = tableballs[i].rad;
   }
   else if (tableballs[i].cx > (width - 1 - tableballs[i].rad)) {
     tableballs[i].vx = -tableballs[i].vx;
     tableballs[i].cx = width - 1 - tableballs[i].rad;
   }

   if (tableballs[i].cy < tableballs[i].rad) {
     tableballs[i].vy = -tableballs[i].vy;
     tableballs[i].cy = tableballs[i].rad;
   }
   else if (tableballs[i].cy > (height - 1 - tableballs[i].rad)) {
     tableballs[i].vy = -tableballs[i].vy;
     tableballs[i].cy = height - 1 - tableballs[i].rad;
   }


   for (int j=0; j<numballs; j++) {

     ColDist = dist(tableballs[j].cx, tableballs[j].cy, tableballs[i].cx, tableballs[i].cy);

     if ( (ColDist <= (clusterMult * tableballs[i].rad)) && (i != j) ) {

       ang = atan2((tableballs[j].cy - tableballs[i].cy), (tableballs[j].cx - tableballs[i].cx));


       //if a ball is on or inside the other, push it to just outside
       dx = -clusterRad * tableballs[i].rad * cos(ang);
       dy = -clusterRad * tableballs[i].rad * sin(ang);
       tableballs[i].cx = dx + tableballs[j].cx;
       tableballs[i].cy = dy + tableballs[j].cy;

       //Get the reflection angle
       if (ang > (2 * PI)) {
         ang = (2 * PI) - ang;
       }
       else if (ang < 0) {
         ang = ang + (2 * PI);
       }

       R = dist(0, 0, tableballs[i].vx, tableballs[i].vy);


       dx = R * cos(ang);
       dy = R * sin(ang);

       //Put the velocity of the 'attacking' ball into the target
       tableballs[j].vx = dx;
       tableballs[j].vy = dy;



       //'Decrease velocity from the attacker
       tableballs[i].vx = 0.2f * tableballs[i].vx;
       tableballs[i].vy = 0.2f * tableballs[i].vy;

       if (magnet == 1) {

         //'Goto first ball
         if (tableballs[0].cx < tableballs[i].cx) {
           tableballs[i].vx = tableballs[i].vx - 1.0f;
         }
         else if (tableballs[0].cx > tableballs[i].cx) {
           tableballs[i].vx = tableballs[i].vx + 1.0f;
         }
         if (tableballs[0].cy < tableballs[i].cy) {
           tableballs[i].vy = tableballs[i].vy - 1.0f;
         }
         else if (tableballs[0].cy > tableballs[i].cy) {
           tableballs[i].vy = tableballs[i].vy + 1.0f;
         }

       }

       // tableballs[i].rad = random(20,60);
       tableballs[i].color1 = color (random (0,255),random (0,255),random (0,255));

     }

   }


   if (i == 0) {
     fill(255);
   }
   else if (i == (numballs - 1)) {
     fill(0,255,0);
   }
   else {
     fill( tableballs[i].color1 );
   }

   ellipse(tableballs[i].cx, tableballs[i].cy,
   tableballs[i].rad,
   tableballs[i].rad);

 }

 stroke(0,255,0);
 // line(tableballs[0].cx, tableballs[0].cy, mouseX, mouseY);

 if (random (999) > 900) {
   mouseClickedFAKE();
 }

}


void mouseClickedFAKE() {
 float R = 0, ang = 0;

 float xWert= random(0,width);
 float yWert= random(0,height);

 R = dist(  xWert, yWert  , tableballs[0].cx, tableballs[0].cy);

 if (R > maxSpeed) {
   R = maxSpeed;
 }

 ang = atan2((yWert - tableballs[0].cy), (xWert - tableballs[0].cx));

 tableballs[0].vx = R * cos(ang);
 tableballs[0].vy = R * sin(ang);

}


class Ball {
 float cx = 0, cy = 0, vx = 0, vy = 0;
 float rad = 40;
 color color1= color (random (0,255),random (0,255),random (0,255));
}

@andrewowaun
At least in Windows (I'm using 2K): yes.
Copy lib folder and the .exe to your windows directory.
Rename the .exe to "yourscreensavername.scr".
Then go to your screen saver settings as usual.
Another trick from VB.

@Chrisir
Entertaining. :]
I think this could be turned into some sort of AI or 'boids' type program.