As a first version of our car cellphone charger we used a circuit like this:

As Gerardo pointed out, this circuit produced a variable voltage output between 5V and 5.5V. We do not desire such behaviour in our cellphone charger since the battery lifespan might be severely reduced.

The main source of such voltage variation lies on the car's voltaje drops between the low and high RPM of the alternator, a phenomenon noticable on old cars (like in my 94' car for instance).

We think that a pair of zeners reverse biased in series could clamp the voltaje at the output in around 5V.

The fuse at the input of the circuit (not shown on schematic) prevents it from passing more than 500mA to the output, that is, our charging cellphone.

On the LM7805 voltage regulator

You must check that the LM7805 regulator you choose can handle currents of about 500mA or more since there is a LM7805 version that can handle up to 100mA. 100mA is a low corruent and the regulator should malfunction due to the high current load. We recommend using the TO220 version.