Motorola's recent selection of Android gadgets (particularly, the Droid Razr) sport a Smart Actions application that automatically modifies your phone settings when certain events occur -- for example, turning off data service and ringer at night, reducing volume when you arrive at work, turn off Bluetooth and Wi-Fi when you get home, etc.
I've seen this in action on my friend's device, and by his account, this is quite a battery-saving and convenience feature.
I wonder if there exists a similar application on the Market for non-Motorola devices.