Constantly changing your phone's settings to match your location, save battery, or otherwise fit your current conditions can get annoying quickly. Llama automates settings changes by detecting factors like where you are, what time it is, and what your phone's doing.
Llama focuses mostly on location, letting you enable certain settings (like Wi-Fi, Bluetooth, volume changes, or starting an app) when you arrive at a specific location, like your home. However, you can also set it to change those settings at other times, like the time of day, when the battery is at a certain level, when a calendar event is near, whether its charging, whether music is playing, or when other conditions are met. So, for example, you could set your phone to automatically switch to vibrate when you leave the house, or automatically turn down your phone's brightness when the battery is getting low.
Many of you will immediately note Llama's similarity to Tasker, an app we've often talked about due to its ability to turn your phone into a fully automated superphone. Llama fulfils a similar purpose, and while it isn't quite as powerful as Tasker, it's free, and has a much simpler UI for beginners. If you want to do something complicated, Tasker's probably still your best bet. But for simple settings changes (like those that'll save your phone's battery), Llama's a great way to go.
Llama is a free download for Android phones running 2.1 and up.