Harbormaster is an official variant for Catan and its expansions, first introduced in Atlantis: Scenarios & Variants. It was created by Klaus Teuber and is later included in Catan: Traders & Barbarians.
The title of Harbormaster is yet another way to earn victory points in any Catan game. It simply adds a new special card, known as "Harbormaster", to the Longest Road and Largest Army special cards. It is played in a similar fashion. The first player to have 3 "harbor points" (settlement or city built on 3 harbors) receives the Harbormaster card and 2 victory points. If another player builds more cities and/or settlements beside harbors, that player then takes the Harbormaster card and it's accompanying victory points. Naturally, this means that the total points that can be earned in a game are increased, thus the points required to win the game should be increased by 1. For example, in a basic game, it takes 11 victory points to win.
A settlement on a harbor gives the player one harbor point. A city on a harbor gives the player two harbor points.