Knights of Honor is a European RTS game (sort of like the Age of Empires games, but not) that I'm now addicted to. It's a medieval style RTS, like a lot of games in the Age of Empires mold, but with less emphasis on military conquest and more on economics, diplomacy, and backstabbing (you can send spies to infiltrate an enemy court and assassinate their king).
My only complaint is that the AI is a bit off-kilter, and it gets a bit unbalanced at times. For example, in one campaign I played through, an AI-controlled Wales once managed to take over all of Great Britain and France, and move on through Germany and Spain. Frigging WALES.
No offense to the Welsh, but you ain't exactly a military powerhouse.
Another time, while playing as Bohemia (basically the old-school Czech Republic), I took over most of Poland, Hungary, and the Balkans - and the rest of the world applauded me for it. I was confused and proud at the same time.
All in all, a fun game that nobody's ever heard of.