Turn on a light when a button is pressed.
For example, there was an instructable on getting a (real, hardware) button to work with a PC running MaxMSP/Puredata: The idea was that you set it up to output an audio tone on one of the earphone channels, put the button in between the audio out and audio-in, and then use an "audio meter object" with a threshold to trigger an event when the button actually connected the tone to the input.
http://www.instructables.com/id/How_to_Connect_a_Push_Button_with_Audio_Input_and_/
It was cheap, effective, and the average PC has plenty of power to do such things. But I was quite boggled...