Various "Free Cities" throughout history

Free City of Danzig

This "free city" is one of the most recent, as its latest form existed between the World Wars. The first Free City of Danzig was created by Napoleon as a client state following his gains from the Treaty of Tilsit, but did not survive Napoleon's fall. Later, after Poland's recreation post-WWI, Danzig became a Free City once again (under control of the League of Nations), on the border between East Prussia and Poland. The Free City did not survive the German invasion of Poland, and its territory was given to Poland after the war (where it is now known as Gdansk), but the Germans who left Poland formed their own government in exile which still officially exists. 

Free City of Krakow

Yet another Polish free city, this one (officially titled the Free, Independent, and Strictly Neutral City of Cracow with its Territory)  was created following the re-partition of Poland after the defeat of Napoleon. While the previous partitions placed Krakow within Austrian territory, the new partition, which created a new Polish state under Russian control, placed Krakow at the corner of Poland, Austria, and Prussia, necessitating the Free City's creation. However, an 1846 uprising led to Austria taking the city, which also happened to create the Three Emperor's Corner.

Holy Roman Empire

The Holy Roman Empire, being composed of smaller substates, included many city-states within its purview. These cities became known as "free imperial cities", and although they did not originally have the same rights as larger kingdoms, later changes to the Holy Roman Empire's structure gave them more power. Even with the dissolution of the Holy Roman Empire, many free cities remained as sub-national entities, and Bremen and Hamburg are still considered "Free (and) Hanseatic Cities" to this day.