Llama gets my vote for the MOST USEFUL Android Application. The idea is simple: the application changes your settings depending on your location.
Practical application: when you leave your house, you can automatically turn off your wifi and background sync (save battery) and set your brightness to auto. When you get to work, you can automatically set your phone to vibrate. When you leave work, you can set it back to normal. It does everything automatically! Handy, right?
Your location is determined by the cell sites around you. So you have to "train" Llama to listen for those cell sites first at your set locations. Then, when Llama detects those cell sites, it automatically sets the settings of your choice. So it doesn't require your GPS, WiFi, or APN to be turned on.
You can download it here: Llama @ Android Market
Llama, by the way, stands for Location Aware Mobile Application. Not sure where the extra L fits in but, hey, I'm not complaining.