Carcassonne is one of my very favorite boardgames. The two expansions "Inns & Cathedrals" and "Traders & Builders" make the game quite fun, indeed. More things to keep in mind, more ways to get points, more ways to subvert others:) I like it most because it's easy to pick up and play a game whenever, but the randomness keeps it interesting.

Settlers of Catan is a great game, as well. I've played both the classic version and with the "Cities & Knights" expansion, which is excellent. This one is more involved and often takes a couple hours with my group of friends. It's fun for a planned night of game-playing and socializing.

I also agree with the recommendation of Puerto Rico. Once you've played it a couple times, you realize the huge number of possibilities for choosing buildings and crops. It also has what I think is a neat twist on the old turned-based game mechanic.

On a related note, if you haven't heard of it yet, you should check out BoardgameGeek.com. It's full of game reviews and recommendations. We've found a number of fun new games through that site.