Here are some apps which may be useful during your stay in Köln

Campus

CampusKoeln App
Everything you need on Campus. For Android and iPhone.
 

Campus navigation

Android, iPhone

Transportation

DB Navigator:
The timetable information by Deutsche Bahn includes all public transportation (rail network, tram, subway, buses).
Android iPhone Website Bahn

KVB App
The app of Kölner Verkehrsbetriebe. Great if you want to use their rent-a-bike system
Android iPhone Website KVB

FlixBus:
The biggest German provider of intercity coach trips in Germany and to other European cities.
Android iPhone Website Flixbus

BlaBlaCar:

Offers a platform to connect to or offer lifts by car to German and European cities.
Android iPhone Website Blablacar

Dictionaries 

dict.cc
Offline Dictionary for many languages
Android iPhone Website dict.cc