I discovered this application after listening to the TWiT network as many of the contributors use this application if they are going to be late or need to see where each of the hosts are. Glympse is a location based application that allows the user to contiously send their GPS location to a select group or with one person.
Some people may not entirely like the idea of sending location but personally I feel this is a good controlled way compared to services like Google Lattitude which can send out publicly. Everything is under your control in Glympse such as duration, sender and the destination.
The app initially starts with some instructions and you can then start your first Glympse. You then select your recipients either by hand or from your contacts list using email (if you have an iPhone you can text). You then set the duration which you can control from 5 minutes to 4 hours depending on how late you maybe or if your on a long trip. Finally you can send a message and send off your Glympse. The Glympse can then be viewed by someone in the app or on the web.
Glympse is obviously an app that requires GPS and an active internet connection and works in the background. It maybe worth thinking before you use this app if you also plan on doing a lot more with your device whilst travelling. I noticed my battery decrease by around 10% for a 1 hour journey whilst not using the iPad.
I highly recommend Glympse and it is available for free as a universal app in the iOS app store and on Android.