Age of Empires Online came to Steam a bit ago and after that really hasn't received as much attention as it should. Its a shame really because its a unique take on the traditional MMO. Its a free to play game available in Steam, so if you haven't checked it out yet, I recommend you do. I reviewed it a bit ago and you can check that out here:

One glaring problem of Age of Empires is how hard it is to install. After you install it in Steam, it requires you to create a Microsoft Live account and obtain an install key to plug in to Age of Empires Online. Creating a Microsoft Live account is trivial, getting the Age of Empires Online key is very hard to figure out but if you read the steps in this thread you should be cooking:

After playing Age of Empires Online recently I have noticed many updates that streamlines the experience and makes things simpler. There are more races to play as and its not hard to buy them or gain points to get them for free. Age of Empires Online keeps all the stuff you love from the Age of Empires 2 core gameplay with a very beautiful graphics overhaul, which is worth the price of admission alone. After the recent set of core upgrades to the gameplay I think giving this game another shot is well worth your time.