This project ilustrates the different contexts for this keyword in nodejs. It's based on the exceltent article Fully Understanding the this Keyword which I higly recommend reading. I also highly recommend the MDN article on this keyword and for a complete understanding of prototype chain, the Yehuda Kat'z article Understanding prototypes in javascript.
As per ECMAScript, when "this" is called outside of any function it references the global context. This situation is a little harder to exploit in node.js since all module calls are embedded in a function to protect the global scope.