Using a cheap microphone and soundcard it is possible to make a computer listen for when someone claps. Clapping is a good signal because its sound is easy to quantify, a short high pitched noise, and so easy to write software to detect. I use this to switch lights on and off when I clap, as well as to play music.
Of course, what I would really like to be able to do is say "Computer, lights!" and make the lights come on. Unfortunately I haven't found any sufficiently accurate speech recognition software for this.
-- pfh