If you want to freak the hell out of someone with a notebook, such as MacBook Pro, install Line-In without their knowledge. Then set it up as I've described above. In its settings, pick MIC for the input and the built-in speakers for the output. Adjust the Sound prefs so that you don't get immediate and painful feedback. Adjust it so that the Mic can just pick up the sound of your typing and quietly echo back the pops and clicks through the speakers.

I can almost guarantee that if someone doesn't know it's been done they're going to freak right out.

They'll also get a nice surprise when playing back audio through their speakers -that concert hall effect.