Traditional greeting in the northern parts of Germany, i.e. the federal states of Hamburg, Schleswig-Holstein, Mecklenburg-Vorpommern (Mecklenburg-Western Pomerania) and parts of Niedersachsen (Lower Saxony).
It is contracted from 'Guten Morgen' (Good Morning) to 'Morgen' to 'Moin'. One important thing to notice though is that it is used at any time of day. This greatly confuses foreigners (such as anyone south of the Elbe river, which is where the Balkans begin in our mind)
Secondly, it is never doubled as 'Moin moin!', except when speaking to tourists.