Nowadays location based applications become more and more popular. Maybe the simplest location based app is sending emergency message with your location. For example, if you go to a place with potential risk, when the accident happens, you might not have enough time to send SMS or make a call to tell where you are exactly.
My little project ttlite is for this: you can preset your SOS message and turn on GPS on your phone, in case the accident happens, you just push one button to send SOS with your location to twitter. You have known about twitter's great power in spreading messages instantly, right?
It is here: http://code.google.com/p/ttlite/