This was my first app. It was how I learned Android app development.
It allows you to set up "hotspots" which, if you go into them than then send an SMS message to a contact. As a cyclist it was so my girlfriend knew where I was and it was also a good way to automatically tell her I was nearly home and put the kettle on.
There's functions to set up emergency hotspots that will send a message if you don't reach that place before a specified time.
It was quite well received and people still use it but other apps such as Google's trusted friends have made it a bit redundant. It did make the pages of my favourite cycling website.