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.
For short trips, using these cars can actually be cheaper than using public transport, especially if you have passengers. Petrol, parking and insurance included in the flat rate. Car2go, DriveNow and Multicity Carsharing all have cars that are found on the streets and that can be rented spontaneously or pre-booked; others require pickup and dropoff at fixed locations.
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.

DriveNow (www.drive-now.com) has a wide range of BMW and Mini models (some of them cabriolets); driving them around costs €0,31-0,34 per minute and registration costs €29. DriveNow is present in Munich, Düsseldorf, Cologne and San Francisco too.

Multicity Carsharing (www.multicity-carsharing.de) uses electric Citroen C-Zero cars. The fee starts at €2,50 per 10 minutes with a maximum fee of €39 per day; registration costs €25. They collaborate with Flinkster (www.flinkster.de), the German Railways car sharing plan, available for €10 per month in 140 German cities as well as in Austria, Switzerland and the Netherlands; this is attractive mainly for BahnCard holders and has flexible rental based around train stations.

Hertz On Demand (www.hertzondemand.com/berlin) has a dozen locations in central Berlin where pre-booked cars can be picked up using a client card from €2 per hour (€1,50 at night and weekends), plus €0,17 per kilometre. Registration is free, and the service is also available in dozens of cities in the US, Canada, UK, Spain and France.

Greenwheels (www.greenwheels.com) charges €5 per month to use cars across Germany (and The Netherlands and the UK) from €2 per hour.