Код:

<script language=JavaScript>
if  ((document.getElementById) && window.addEventListener || window.attachEvent)

{(function()

{
var num = 30;   // Количество снежинок
var timer = 30; // Скорость падения
var enableinNS6 = 1; 
var y = [];
var x = [];
var fall = [];
var theFlakes = [];
var sfs = [];
var step = [];
var currStep = [];
var h,w,r;
var d = document;
var pix = "px";
var domWw = (typeof window.innerWidth == "number");
var domSy = (typeof window.pageYOffset == "number");
var idx = d.getElementsByTagName('div').length; 
for (i = 0; i < num; i++) {

 sfs[i] = Math.round(1 + Math.random() * 1);
 document.write('<div id="flake'+(idx+i)+'" style="position:absolute;top:0px;left:0px;width:'+sfs[i]+'px;height:'+sfs[i]+'px;background-color:#ffffff;font-size:'+sfs[i]+'px"></div>');
 currStep[i] = 0;
 fall[i] = (sfs[i] == 1)?Math.round(2 + Math.random() * 2): Math.round(3 + Math.random() * 2);
 step[i] = (sfs[i] == 1)?0.05 + Math.random() * 0.1 : 0.05 + Math.random() * 0.05; 

 }
 if (domWw) r = window;
  else { 
   if (d.documentElement && typeof d.documentElement.clientWidth == "number" && 
  d.documentElement.clientWidth != 0) r = d.documentElement;
    else { 
     if (d.body && typeof d.body.clientWidth == "number")  r = d.body;
   }
  }
function winsize() {

 var oh,sy,ow,sx,rh,rw;
 if (domWw) {
  if (d.documentElement && d.defaultView && typeof d.defaultView.scrollMaxY == "number") {
   oh = d.documentElement.offsetHeight;
   sy = d.defaultView.scrollMaxY;
   ow = d.documentElement.offsetWidth;
   sx = d.defaultView.scrollMaxX;
   rh = oh-sy;
   rw = ow-sx;
 }
  else {
   rh = r.innerHeight;
   rw = r.innerWidth;
  }
 h = rh - 2;  
 w = rw - 2; 

 } 
 else{ 

 h = r.clientHeight - 2; 
 w = r.clientWidth - 2; 
}
}

function scrl(yx) { 

 var y,x;
 if (domSy) {
  y = r.pageYOffset;
  x = r.pageXOffset; }

 else { 

 y = r.scrollTop;
  x = r.scrollLeft;

 }

 return (yx == 0)?y:x;

}
function snow() {

 var dy,dx;
 for (i = 0; i < num; i++) { 

  dy = fall[i];
  dx = fall[i] * Math.cos(currStep[i]);
  y[i]+=dy;
 x[i]+=dx; 
  if (x[i] >= w || y[i] >= h) {

   y[i] = -10;
   x[i] = Math.round(Math.random() * w);
   fall[i] = (sfs[i] == 1)?Math.round(2 + Math.random() * 2): Math.round(3 + Math.random() * 2); 
   step[i] = (sfs[i] == 1)?0.05 + Math.random() * 0.1 : 0.05 + Math.random() * 0.05 ;
 }

 theFlakes[i].top = y[i] + scrl(0) + pix;
 theFlakes[i].left = x[i] + scrl(1) + pix;
 currStep[i]+=step[i];

}

setTimeout(snow,timer);

}
function init() {

 winsize();
 for (i = 0; i < num; i++) {

  theFlakes[i] = document.getElementById("flake"+(idx+i)).style;
  y[i] = Math.round(Math.random()*h);
  x[i] = Math.round(Math.random()*w); 

 }

 snow();

}
if (window.addEventListener) {

 window.addEventListener("resize",winsize,false);
 window.addEventListener("load",init,false);

} 
else 
 if (window.attachEvent) {

  window.attachEvent("onresize",winsize);
  window.attachEvent("onload",init);

}  


})();
}
</script>