I grabbed Age of Empires Online through Steam the other day. Its a free to play with a bunch of pay DLC available. There is a decent amount of content to the base game and I was having a ton of fun with it so I ended up grabbing the whole DLC pack on Steam. 66% off the normal price so it was 16.99 instead of $50.00.
I put about 4.5 hours into it yesterday evening. It is an interesting mix of genres since you have the normal AoE RTS with a bit of an MMO built around it. The time period that the game is in is closer to the first two AoE games, while the idea of home cities from AoE3 is where the MMO part comes into play. This time around though you actually get to manage and build up your home city instead of just leveling to unlock special abilities like in AoE3. In your home city, as well as different cities around the world map you'll find NPCs who give you quests (the campaign style missions for the RTS part of the game). After you get through the first few tutorial missions many quests can be played cooperatively (you can skip to city level 3 if you don't want the tutorial, but you miss out on some easy resources that way). When you complete a quest successfully you are given rewards and experience as you would expect in an MMO. The resources generated and earned at the home city level can be used to expand your city and eventually to craft gear and consumables that make your units/buildings more powerful. As you gain levels you also get points to spend in your tech tree.
If you're a fan of the AoE style RTS then you should definitely check this game out. The early quests are a bit on the easy side, but the game should be a lot of fun and the later quests allow you to play in Elite mode for more of a challenge and better rewards.