Perhaps the smallest and least known principality: the Principality of Seborga, in the hinterland go
Under the authority of the Templars Seborga was an independent state after which it became a monasticism. In the year 1079 Seborga was inducted as a principality under the Holy Roman Empire. Although it was later bought by the Savoy family this act was never found and for this Seborga can still be officially called a principality.
Today, the "reigning prince" was democratically elected but Seborga still has its own currency, its own stamps, a flag and anthem and the cars have their own license plate with the denotation SB.