Make your iPhone ... fart. Provide you more than 20 fart sound effects and have count down function to play the sound you specify automatically.
Keep it in your pocket and see everyone move away from you (really useful in the crowded places, such as in the buses and trains). You can also set the countdown and put the iPhone under a friend's chair. Gather all your friends and laugh to tears when the poor guy starts farting. Imagine his face!