ECMA Script for CreateSite

var clientContext = null;
var web = null;
ExecuteOrDelayUntilScriptLoaded(Initialize, "sp.js");
function Initialize()
{
clientContext = new SP.ClientContext.get_current();
web = clientContext.get_web();
var webCreateInfo = new SP.WebCreationInformation();
webCreateInfo.set_description("All about Rare solutions.");
webCreateInfo.set_language(1033);
webCreateInfo.set_title("Rare Solutions - SharePoint and .NET");
webCreateInfo.set_url("RareSolutionsSharePoint");
webCreateInfo.set_useSamePermissionsAsParentSite(true);
webCreateInfo.set_webTemplate("BLOG#0");

this.oNewWebsite = this.web.get_webs().add(webCreateInfo);

clientContext.load(this.oNewWebsite, 'ServerRelativeUrl', 'Created');

clientContext.executeQueryAsync(Function.createDelegate(this, this.onCreateWebSuccess),

Function.createDelegate(this, this.onQueryFailed));
}
function onCreateWebSuccess(sender, args) {
alert("Web site url : " + this.oNewWebsite.get_serverRelativeUrl());
}

function onQueryFailed(sender, args) {
alert('request failed ' + args.get_message() + '\n' + args.get_stackTrace());
}