Many Berliners already have a monthly ticket, use their bicycles or cars to get around. For visitors, we recommend to get a BVG day ticket or a small group ticket for up to 5 persons. More on the tickets can be found here: BVG Tickets
The BVG also has developed a free app for Android, Windows and iPhone where you can check different routes, see maps and even buy tickets. It allows you to use your mobile phone as a ticket and find out about connections or save favourite routes and call them up at the tap of a finger. In addition, you can display the locations of available car club vehicles from Car2go with an interactive area view. You can read more about it and download it here: http://www.bvg.de/de/Fahrinfo/Jederzeit-mobil
You can also get around using car sharing. If you use different car sharing providers, you can use Free2Move app to organise your cars. Free2Move makes it easy for you to book vehicles from different providers, so you don´t have to worry about switching back and forth between carsharing applications.
Available for iPhone, Android and Windows Phones. Check it out here: https://de.free2move.com/en/app