Hola prueba este código, insertalo en body

<script language="JavaScript">
var no = 49;
var speed = 25;
var snowflake = "http://img.webme.com/designs/globals/snow.gif";
</script>
<script language="JavaScript" src="http://img.webme.com/designs/globals/snow.js"></script>

Este es otro, nieva al mover el ratón 

<script type="text/javascript">
// <![CDATA[
var colour="#f3e9e9";
var sparkles=50;/*************

********************************************/
var x=ox=400;
var y=oy=300;
var swide=800;
var shigh=600;
var sleft=sdown=0;
var tiny=new Array();
var star=new Array();
var starv=new Array();
var starx=new Array();
var stary=new Array();
var tinyx=new Array();
var tinyy=new Array();
var tinyv=new Array();

window.onload=function() { if (document.getElementById) {
var i, rats, rlef, rdow;
for (var i=0; i<sparkles; i++) {
   var rats=createDiv(3, 3);
   rats.style.visibility="hidden";
   document.body.appendChild(tiny[i]=rats);
   starv[i]=0;
   tinyv[i]=0;
   var rats=createDiv(5, 5);
   rats.style.backgroundColor="transparent";
   rats.style.visibility="hidden";
   var rlef=createDiv(1, 5);
   var rdow=createDiv(5, 1);
   rats.appendChild(rlef);
   rats.appendChild(rdow);
   rlef.style.top="2px";
   rlef.style.left="0px";
   rdow.style.top="0px";
   rdow.style.left="2px";
   document.body.appendChild(star[i]=rats);
}
set_width();
sparkle();
}}

function sparkle() {
var c;
if (x!=ox || y!=oy) {
   ox=x;
   oy=y;
   for (c=0; c<sparkles; c++) if (!starv[c]) {
     star[c].style.left=(starx[c]=x)+"px";
     star[c].style.top=(stary[c]=y)+"px";
     star[c].style.clip="rect(0px, 5px, 5px, 0px)";
     star[c].style.visibility="visible";
     starv[c]=50;
     break;
   }
}
for (c=0; c<sparkles; c++) {
   if (starv[c]) update_star(c);
   if (tinyv[c]) update_tiny(c);
}
setTimeout("sparkle()", 40);
}

function update_star(i) {
if (--starv[i]==25) star[i].style.clip="rect(1px, 4px, 4px, 1px)";
if (starv[i]) {
   stary[i]+=1+Math.random()*3;
   if (stary[i]<shigh+sdown) {
     star[i].style.top=stary[i]+"px";
     starx[i]+=(i%5-2)/5;
     star[i].style.left=starx[i]+"px";
   }
   else {
     star[i].style.visibility="hidden";
     starv[i]=0;
     return;
   }
}
else {
   tinyv[i]=50;
   tiny[i].style.top=(tinyy[i]=stary[i])+"px";
   tiny[i].style.left=(tinyx[i]=starx[i])+"px";
   tiny[i].style.width="2px";
   tiny[i].style.height="2px";
   star[i].style.visibility="hidden";
   tiny[i].style.visibility="visible"
}
}

function update_tiny(i) {
if (--tinyv[i]==25) {
   tiny[i].style.width="1px";
   tiny[i].style.height="1px";
}
if (tinyv[i]) {
   tinyy[i]+=1+Math.random()*3;
   if (tinyy[i]<shigh+sdown) {
     tiny[i].style.top=tinyy[i]+"px";
     tinyx[i]+=(i%5-2)/5;
     tiny[i].style.left=tinyx[i]+"px";
   }
   else {
     tiny[i].style.visibility="hidden";
     tinyv[i]=0;
     return;
   }
}
else tiny[i].style.visibility="hidden";
}

document.onmousemove=mouse;
function mouse(e) {
set_scroll();
y=(e)?e.pageY:event.y+sdown;
x=(e)?e.pageX:event.x+sleft;
}

function set_scroll() {
if (typeof(self.pageYOffset)=="number") {
   sdown=self.pageYOffset;
   sleft=self.pageXOffset;
}
else if (document.body.scrollTop || document.body.scrollLeft) {
   sdown=document.body.scrollTop;
   sleft=document.body.scrollLeft;
}
else if (document.documentElement && (document.documentElement.scrollTop || document.documentElement.scrollLeft)) {
   sleft=document.documentElement.scrollLeft;
sdown=document.documentElement.scrollTop;
}
else {
   sdown=0;
   sleft=0;
}
}

window.onresize=set_width;
function set_width() {
if (typeof(self.innerWidth)=="number") {
   swide=self.innerWidth;
   shigh=self.innerHeight;
}
else if (document.documentElement && document.documentElement.clientWidth) {
   swide=document.documentElement.clientWidth;
   shigh=document.documentElement.clientHeight;
}
else if (document.body.clientWidth) {
   swide=document.body.clientWidth;
   shigh=document.body.clientHeight;
}
}

function createDiv(height, width) {
var div=document.createElement("div");
div.style.position="absolute";
div.style.height=height+"px";
div.style.width=width+"px";
div.style.overflow="hidden";
div.style.backgroundColor=colour;
return (div);
}
// ]]>
</script>
 

Un saludo.