HUE HUE FANNEY!
yeh, it's car2go, a car sharing company owned by Smart. They offer cars in a lot of big cities (not only in Germany), you can just enter a free one and drive within the city boundaries and pay by minutes. You enter with an electronic card, inside you find the key and unlock it again with a pin. Pretty simple and useful.
I pay around 5 Euros for a drive from work to home, which is about 20 minutes.