A single global with a namespace method

namespace.js

//BSD Licensed

var YourGlobal = {
    namespace: function(ns){
        var parts = ns.split("."),
            object = this,
            i, len;
        
        for (i=0, len=parts.length; i < len; i++) {
            if (!object[parts[i]]) {
                object[parts[i]] = {};
            }
            object = object[parts[i]];
        }
        
        return object;
    }
};

//usage
YourGlobal.namespace("foo.bar");
YourGlobal.foo.bar.message = "Hello world!";

YourGlobal.namespace("foo").baz = true;