Llama or Tasker for Android is pretty fucking nuts if you get into the scripts you can program into it. I use it for simple shit like
-Whenever I plug something into the headphone jack, Podcast Addict loads up
But people really go nuts and you can have it do shit that just blows my brain like:
-Use GPS to auto-send a text to a certain person (say, your wife) when you're a predetermined distance from home. "Be there in 10". Phone doesn't even leave your pocket.
-Opens a grocery list you have stored in Notes when you walk into the grocery store.
-Wifi on when at home and work, off everywhere else;
-3G off when Wifi on and vice-versa;
-Set noisy contacts, even if the phone is in vibration, a small beep is played for them
-Lower multimedia/notifications volume with headphones on (sound too loud when at full volume);
-Auto rotation on for certain apps (gallery, browser, etc), off for everything else;
-Time based rules (silent at night, no data at night, no bluetooth at weekends, etc)
-Connect bluetooth between 8:30-10:30 and 16:30-17:30 (commutes)
-Disable all data when battery lower than 5% (wifi, bluetooth, gps, 3g, etc)
-Someone tries to unock your phone with the wrong password 3x, a picture is taken with the front camera and emailed directly to you, with GPS coordinates.
Those are just some from reddit posters, the list goes on and on.
Technology! In your pocket!