Implementation of observer pattern for Node.js
The name of the Observer class is confusing: an Observer contains observers?  I think that the class would make more sense if you renamed it Observable.

For the .on() and .off() methods, I think that the event parameter should come first.  That would mimic the natural English expression: "on event tell observer to do action".  It's also how it's done in other frameworks like jQuery.

I'm mildly annoyed by the excessive vertical spacing of the code.