The Mac OS X terminal has a special command called say that allows you to make your computer talk. Just open a terminal window (Applications -> Utilities -> Terminal) and type say followed by what you want your computer to say. For example say this is a test. Once you hit Enter, your computer will actually speak the words that you entered after the say command.

I'm not sure what application this has, but there must be some fiendishly clever way to play a trick on your friends using this command.