What iPhone app will allow one person to track the location of another in real-time? With the permission of the person being tracked, of course!

I'm running in a marathon and want my family and friends to be able to see my location on a map in real-time (or very close to it). 

Are there any apps that can do this?  

I'd also like to be able to turn on and off the "broadcast" of my location.
Find my Friends by Apple.

More info here.