Código AJAX (ajax.js):

var cadena=location.href;
var pos=cadena.lastIndexOf('#')
document.write("<p>Espere mientras carga la página.</p>");
if (pos > 1){
 var url=cadena.substr(pos+1);
 loadurl(url+'.html', 'main');
}
else{
 loadurl('pagina1.html', 'main');
}

function loadurl(url, id){
	var cadena=location.href;
	var pos=cadena.indexOf('#')
	var posurl=url.indexOf('.')
	location.href=cadena.substr(0,pos)+'#'+url.substr(0,posurl);
	var pagecnx = createXMLHttpRequest();
	pagecnx.onreadystatechange=function() {
		if (pagecnx.readyState == 4 && (pagecnx.status==200 || window.location.href.indexOf("http")==-1))
		document.getElementById(id).innerHTML=pagecnx.responseText;
	}
	pagecnx.open('GET', url, true);
	pagecnx.send(null);
	return false;
}

function createXMLHttpRequest() {
	var xmlHttp=null;
	if (window.ActiveXObject) 
		xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
	else if (window.XMLHttpRequest) 
			xmlHttp = new XMLHttpRequest();
	return xmlHttp;
}