An over at IBM developerWorks an article has been posted titled Whistle while you work to run commands on your computer:

"Use Linux® or Microsoft® Windows®, the open source sndpeek program, and a simple Perl script to read specific sequences of tonal events -- literally whistling, humming, or singing at your computer -- and run commands based on those tones. Give your computer a short low whistle to check your e-mail or unlock your your screensaver with the opening bars of Beethoven's Fifth Symphony. Whistle while you work for higher efficiency."

<joke>Be warned, if your microphone picks up your co workers humming or mumblings it may cause unexpected results :P</joke>

Update: See also Knock-based commands for your Linux laptop