Plug In Launcher is a free Android app that does just one thing, and it does it well -- it launches any app of your choice when it detects power or a connected pair of headphones. In fact, it can pair one app to your phone's 3.5mm headphone jack and another to your USB port, letting it launch, say, a music player when you plug in headphones, and perhaps an alarm clock while you're charging it overnight, saving you a button press (note: the "Would you like to launch" message is optional) each time. All it asks in return is a pair of running processes that eat up 5.7MB of memory (as of this writing) and the ability to restart itself when you reset your phone. Sure, the app's a little limited compared to context-aware suites like Locale
or Nokia Situations
, but free is free, and this one's useful.