Aquí os dejo un codigo para hacer un efecto de nieve a vuestra pagina.
El codigo es:

<script language="JavaScript1.2">

//Configura la URL de la imagen
var snowsrc="snow.gif"
//Selecciona cuantas imagenes quieres que aparezcan
var no = 10;

var ns4up = (document.layers) ? 1 : 0; // browser sniffer
var ie4up = (document.all) ? 1 : 0;
var ns6up = (document.getElementById&&!document.all) ? 1 : 0;

var dx, xp, yp; // coordinate and position variables
var am, stx, sty; // amplitude and step variables
var i, doc_width = 800, doc_height = 600;

if (ns4up||ns6up) {
doc_width = self.innerWidth;
doc_height = self.innerHeight;
} else if (ie4up) {
doc_width = document.body.clientWidth;
doc_height = document.body.clientHeight;
}

dx = new Array();
xp = new Array();
yp = new Array();
am = new Array();
stx = new Array();
sty = new Array();

for (i = 0; i < no; ++ i) {
dx[i] = 0; // set coordinate variables
xp[i] = Math.random()*(doc_width-50); // set position variables
yp[i] = Math.random()*doc_height;
am[i] = Math.random()*20; // set amplitude variables
stx[i] = 0.02 + Math.random()/10; // set step variables
sty[i] = 0.7 + Math.random(); // set step variables
if (ns4up) { // set layers
if (i == 0) {
document.write("<layer name="dot"+ i +"" left="15" top="15" visibility="show"><a href="http://dynamicdrive.com/"><img src='"+snowsrc+"' border="0"></a></layer>");
} else {
document.write("<layer name="dot"+ i +"" left="15" top="15" visibility="show"><img src='"+snowsrc+"' border="0"></layer>");
}
} else if (ie4up||ns6up) {
if (i == 0) {
document.write("<div id="dot"+ i +"" style="POSITION: absolute; Z-INDEX: "+ i +"; VISIBILITY: visible; TOP: 15px; LEFT: 15px;"><a href="http://dynamicdrive.com"><img src='"+snowsrc+"' border="0"></a></div>");
} else {
document.write("<div id="dot"+ i +"" style="POSITION: absolute; Z-INDEX: "+ i +"; VISIBILITY: visible; TOP: 15px; LEFT: 15px;"><img src='"+snowsrc+"' border="0"></div>");
}
}
}

function snowNS() { // Netscape main animation function
for (i = 0; i < no; ++ i) { // iterate for every dot
yp[i] += sty[i];
if (yp[i] > doc_height-50) {
xp[i] = Math.random()*(doc_width-am[i]-30);
yp[i] = 0;
stx[i] = 0.02 + Math.random()/10;
sty[i] = 0.7 + Math.random();
doc_width = self.innerWidth;
doc_height = self.innerHeight;
}
dx[i] += stx[i];
document.layers["dot"+i].top = yp[i];
document.layers["dot"+i].left = xp[i] + am[i]*Math.sin(dx[i]);
}
setTimeout("snowNS()", 10);
}

function snowIE_NS6() { // IE and NS6 main animation function
for (i = 0; i < no; ++ i) { // iterate for every dot
yp[i] += sty[i];
if (yp[i] > doc_height-50) {
xp[i] = Math.random()*(doc_width-am[i]-30);
yp[i] = 0;
stx[i] = 0.02 + Math.random()/10;
sty[i] = 0.7 + Math.random();
doc_width = ns6up?window.innerWidth : document.body.clientWidth;
doc_height = ns6up?window.innerHeight : document.body.clientHeight;
}
dx[i] += stx[i];
if (ie4up){
document.all["dot"+i].style.pixelTop = yp[i];
document.all["dot"+i].style.pixelLeft = xp[i] + am[i]*Math.sin(dx[i]);
}
else if (ns6up){
document.getElementById("dot"+i).style.top=yp[i];
document.getElementById("dot"+i).style.left=xp[i] + am[i]*Math.sin(dx[i]);
}
}
setTimeout("snowIE_NS6()", 10);
}

if (ns4up) {
snowNS();
} else if (ie4up||ns6up) {
snowIE_NS6();
}

</script>



Es posible utilizar otra imagen cambiando la linea

var snowsrc="snow.gif"

Aqui os dejo 3 tipos de imagenes que podeis usar, podeis guardarlos a vuestro ordenador haciendo click con el boton derecho y en Guardar imagen como...

Artículo por cortesía de Eloi de San Martín
www.programacionweb.net