Is there a bus tracking app?

I was thinking that this must have been thought of/invented already cos it seems so obvious. An app that tracks exactly where a bus is so you know how much time you need to get to the bus stop, whether you will make it, whether you have that extra minute to do something before you dash out the door if its running. If at london bus stops it can tell you how long the bus will be - Why cant it send it to a phone, I mean buses and phones have sat nav so everything is already in place! Does anyone know if this exists? Thanks!