Puerto Rico, Power Grid, Race for the Galaxy >> Settlers of Catan, all the way.
Basically, start at the top and go down: http://www.boardgamegeek.com/browse/boardgame
Basically, start at the top and go down: http://www.boardgamegeek.com/browse/boardgame
Settlers, Bohnanza and Carcassonne are excellent starters into the world of boardgames. Geeks will want to graduate to Caylus, Puerto Rico and the like later on.