So many good ones.
Diplomacy is a great classic dice-less game but you really need 7 people and a lot of time to play it. One of those games that can work well in an online\correspondence model as you need a bunch of time for negotiating during each turn.
Settlers of Catan is a nice intro to the world of German board games. The rules are simple enough for casual gamers to learn but the interaction between players and multiple paths to victory give it a lot of depth and replayability. This game is so popular it has been ported to Xbox live. The previously mentioned Carcassonne is another fun German board game which has been ported to Xbox live.
My personal favorite is Puerto Rico. Multiple paths to victory means lots of different strategies and interaction between players. Requires a few games to get the feel, but has a ton of replayability due to all the potential strategies and the ways these strategies can interact.