Of course you can also attach other inputs - such as the light dependent resistor describer in the $50 robot.
But here is my own output device which I find really useful for testing:-
Its a 3 pin plug - that plugs into your $50 robot headers - and has an LED and resistor wrapped up in insulator tape. You can then plug it into your board to check for high/low outputs or as a power on indicator which could then be removed from your final design. So its a kind of 'visual' continuity tester?