Tasker is the most powerful app in Android. It is a GUI base script engine for Android. User can performs different actions based on different trigger events, pretty much anything that the Android OS supports. With Tasker, you write a small script in 2 minutes which pretty much does the job of a silly app that costs $2.
Here are example some useful scripts idea for example. The phone can read the WiFi status to figure out I am at work and turn down the ringer volume. The phone will turn off 3G data and WiFi when the screen is off to save power, yet still kick off auto-sync every 30 minutes with data connection. Whenever I plug in the headphone, the phone will pop up a menu let me select which music player to launch and auto-play the music.
Here is the best script I have created so far. In case I lost my phone, I can sent an SMS to my phone. It will turn on GPS, get its location, use Google Map to convert GPS data into human readable address and send it back to me via SMS.
With Tasker, I can turn my phone into a fully automated personal assistant. In fact Tasker is the app that set Android apart from iPhone and represent the difference in design philosophy between iPhone and Android. Apple spoon feed iPhone user but never let them step outside of the fence. Google gives average Android users a phone close enough to iPhone, but it allows power users fully customize his phone to make it fun and productive. Once you had experienced the freedom and power of Android, you will never go back to iPhone.