Usually Extended Event Handlers is a good starting point. It doesn't noticably add anything to the game, but its 'under-the-bonnet' functionality is not to be sniffed at.
What does it add to the game by default? As far as I know nothing. All it does is adds the possiblity for custom addons to have eventhandler functionality without them conflicting with eachoter (which it does when your custom addons are not made extended eventhandler complient).
Or, did I miss something there?