German license plates are a necessity. The most common are the long narrow ones that look best on the front bumper. There are square plates that are made and people put these on the decklid where the real license plate should be, (then they put the real plate on the bumper). The square ones are used in Germany for mopeds. Sometimes you can get a matching set of rectangular plates for about $30...that's a sweet deal.
Be careful when buying a plate, there are two kinds. The kind that actually came from Germany and the manufactured aftermarket kind. You can tell which is which because the German ones have a reflective coating on top so it can be seen at night. They also have little round stickers on them that will be scratched out. The German government does this when it cancels a plate.
The reflecting foil on the plates was mandatory since about 1995. Before that, license plates were also available as non-reflecting plates. Original German plates can be detected by some small engraved logos on the back (and on the top of the reflecting ones.) One of the two plastic stickers (looks the same on both front and rear plates) represents the county where the car was registered, this one will most likely be scratched out. The round colored sticker indicates the date when the car has to go through the next technical control, usually every two years for non-commercial passenger cars. The other one indicates the date for the next smog-check (but only cars after 1969 have to be checked). Usually a set of brand new plates upon registration of the vehicle costs around $40 US.
Oval plate isn't as popular, but still cool. They were used when you wanted to export your car out of Germany and into another country. Nowadays, they have a new plate which looks like the original German ones.
Switzerland plates are new, at least in Europe. They are similiar to German plates and are a good way to go against the crowd.
