How can I get specific SharePoint site GUID using javascript/jquery

Use get_id to get the GUID

var value = SP.Web.get_id();

Example usage:

<script type="text/javascript">  
ExecuteOrDelayUntilScriptLoaded(getSite, "sp.js");  
var context = null;  
var web = null;  
    function getSite() {  
        context = new SP.ClientContext.get_current();  
        web = context.get_web();  
        context.load(web);  
        context.executeQueryAsync(Function.createDelegate(this, this.onSuccessMethod), Function.createDelegate(this, this.onFailureMethod));  
    }  
    function onSuccessMethod(sender, args) {  
        alert('your are there :' + web.get_title() + '\n ID:' + web.get_id());  
    }  
    function onFaiureMethodl(sender, args) {  
        alert('you suck ' + args.get_message() + '\n' + args.get_stackTrace());  
    }  
</script>