If you are out on a ride with mates that also have smart phones, then you best download the app called Find Friends, that way if one of you's gets lost, the lost party can send you his position, and you received his exact latitude, longitude and can find him / her easily.

You should have Google Maps better than the Ordnance Survey Maps these days.

If you want the ultimate Sat Nav app its not free but cheaper than a Garmin (tenner) and that is CoPilot Live. very very good.

If you live in London and your bike is kaput, and you are left on foot, there's an app that tells you what bus / train to catch to get you back home and even points out on the map how to get to the bus stop and that app is: CITYMAPPER London

If you are with the AA recovery then you ought to download the AA App because if you breakdown in the middle of nowhere at night and you don't know exactly where you are, the app send AA your exact location so they'll find you.

Also get an app called Around Me, once again using Satellite pointers, it will find you the nearest, restaurant, petrol station, chemist, mechanic whatever. you just search for it and the app will list you all the options near you

I can't think of anything else...