diablolita says: I was only in Berlin for a week - in the late 90's, but it's definitely my favorite city in Germany and I've been to all the major German cities. These apps are helping me plan my next trip there.
Germany Subway & Bus This app shows you all the subways, buses, and stops near you using GPS. It's fast, and the interface is simple. Great info to have - works for all major cities in Germany.
Berlin Metro This is a comprehensive interactive guide to the Berlin train and subway system. It has a full map and zoom, plus full offline functionality. The map is not always current, however - so be sure to check it.
Berlin U-Bahn AR In case you go to Berlin for Oktoberfest and are so drunk you need more help navigating the rail system than any other app can give you - here is an augmented reality map that will physically guide you through Berlin U-Bahn lines.
TravelBook Berlin This is a great offline travel guide for Berlin - like having a free guidebook on your phone! It has a map, routing, and directions all without using internet. Also wikipedia entries on different locations stored. Nice app!
Berlin Parking This app lets you loo at free and metered on street parking in Berlin. It tracks you via GPS and shows more than 500 parking areas around Berlin including prices. VERY useful for driving and parking in Berlin!