Some exotic heresies that appeared in Southeastern Europe esp. in Byzantium and the Balkan region (XIV century)

Adamism (my favourite ): The adamits ( people who profess that heresy) refused to wear any clothing. They did it because they were against any social differences.
Varlaamism (not sure about the spelling, the name came from the first leader Varlaam of Calabria): They were rivals of the isichasts (who believed that God can not became known by using the mind but by devotion) and they believed in the logical explanation of the God's nature. However the isichasm became an official doctrine of the Orthodox church and the varlaamits were excommuncated.

Islam

Hashashins (or Assasins)-existed in Iran in the Middle ages. they were mentioned above as Assasins but the original name is Hashashins. They were ruled by a misterious person, called the Old man ( that was a title not only one person). His followers were taking opium and had to do everything what their master ordered. Most often that was an assassinations of people who weren't liked by the organisation.