I am a teacher in a coaching centre. My students answer the multiple choice questions and send them to me through SMS. My mobile phone has an app which will correct these answers and send the students their marks within two seconds. This is the most valuable app for me when I am on the road.
Another Android app SMSInvite enables me to communicate to my students about the class timings, scheduled classes, postponement or cancellation of classes when I am on the road.
Of course the GPS is important on the road in case I want to know an unknown place.