Catan was my first German-style board game[0]. I've since "graduated" on to more complicated games, so I rarely play it anymore, but I find that it's an effective "gateway" German-style board game. Almost all of my friends who have played it love it, and have gone on to try (and enjoy) other German-style games. Catan is nice because it's sufficiently more complex (and well-designed) than Monopoly so as to be interest…
The Settlers video game is a more complex and is more like Age of Empires (a few years later) but with a focus on low level business and building up of an economy. You need a forester, lumberjack and a sawmill to get planks. You need a well, a corn farm, a corn mill and a baker to get bread. But the basic goal is to expand your territory like years later in Catan. Anno [2] video game series is very similar to The Settlers with a focus on trading ships and islands.
[0] http://en.wikipedia.org/wiki/The_Settlers_of_Catan