There are several expansions and additional rules that can be added to this wonderfully simple game, but for everything you can add, the basic game is still amazing and well worth picking up for game night. Since you can set up the board with a near infinite number of variations, you can play it over and over again, and once you've worn through enough of the basics, you can add the 5-6 player expansion, Seafarers of Catan, Cities and Knights for even more complexity.
Settlers of Catan is probably one of my very favorite board games, and a great addition to every home. It teaches kids about resource management and strategy, and makes for a subtle teaching method. Intended for children ages 10 and up due to small parts.