Finding your way around the city....

From our awesome guide Arjan:

For Android users, there's a free map app called "MapsWithMe" that can be used offline - hence no data connection necessary while in Germany. Just download the Berlin package once you installed the app, enable GPS, and you have an offline 'GoogleMaps' with you.

https://play.google.com/store/apps/details?id=com.mapswithme.maps&hl=en. There is no iPhone version... but this app might do the same trick (and in English): https://itunes.apple.com/en/app/fahrplan/id284970116?mt=8

For public transport planning, I can recommend the free app Öffi (in English) that actually covers many cities. It also has a Berlin metro map and surrounding maps of big stations like Alexanderplatz or the central train station. This app does need an Internet connection, so you should use it with a wifi connection (hotel, café) to avoid roaming costs.
https://play.google.com/store/apps/details?id=de.schildbach.oeffi&hl=en