As Berlin has excellent public transport and good bike paths, most Berliners don't own a car, and most visitors don't need one. But for trips away from the city centre, a set of wheels can be very handy and apart from the traditional car rental companies, there are now several schemes that allow for short rentals. They're especially useful for people already signed up to sister schemes abroad, or regular visitors to Berlin.
Registering often involves visiting an office for an ID check and picking up a client card, but once you have that, you can often use the network in other cities and countries too.
Car2go (www.car2go.com) has nifty Smart cars in Berlin, Düsseldorf, Hamburg, Köln, Ulm and a dozen cities in the US, Canada, UK, Netherlands, Austria and France. Registration (€19, includes 30 free minutes) can be done at their Berlin shop at Rosa-Luxemburg-Straße 2 (or some Europcar offices), after which you can drive off, paying €0,29 per minute, with a maximum of €12,90 per hour.