Interesting stuff. In the US, every state and territory has its own rules, and its own design. In addition to the standard plate, most states offer various vanity or commemorative plates - my California plate is both a vanity plate and a "save the whales" plate (part of the fee goes towards oceanic environmental protection). Washington State's standard plate used to be three alpha and three digits, the alpha designated the county the owner resides in.

Edited to add a link to my license plate clicky