The iPhone presents a constant dilemma: do you put your ringer on loud so you can hear it or do you keep it on silent to not annoy the people around you? PocketMode makes the decision for you.
PocketMode isn't really an app, it's a settings tweak available for jailbroken iPhones. It smartly uses your iPhone's ambient light sensor to determine where your iPhone is. So if it's in your pocket (dark), the ringer is customized to be louder than when it's on your desk (bright). And don't worry, there's a night mode to prevent it from shrieking during your sleep.
Because I'm a lazy and forgetful human being. But I try to be courteous! I always get startled by loud ringtones so I keep my iPhone depressingly on silent to avoid death stares from co-workers. That's okay when I'm on a desk but I end up forgetting to switch back and miss tons of phone calls. Lose, lose. PocketMode is smart enough to use the light sensor to do the switch for me. It's a feature that Apple should seriously consider implementing and a freedom more often seen on Android.