Droid inPocket Premium

Set it and forget it: your phone will get smarter for free.
Auto-adjust your ringer/vibration.
inPocket will set your phone to vibration mode when the phone enters your pocket or when it is facing down.
When out-of-pocket, ringer volume will adjust automatically to ambient noise level: if it's quiet volume will be minimum and when on a loud place, the subway for example, it will try to be audible.

For headphones and headsets users, Droid inPocket has a mode called headPhones, capable of reading out loud any received sms message or incoming call contact/number.