If you plug your iPhone into a stereo or PA system to play music at a party, the Megaphone app turns the iPhone into a microphone. Launch the app, and make a speech. Get everyone to wave their hands in the air like they just don’t care. Pair it up with a wireless bluetooth headset, and you can continue to DJ hands free.
The reviews in the App Store seem to indicate it works as advertised. Megaphone recently saw a price drop from $1.99 to Free. Get it while it lasts (App Store link).