Crusader Kings II is going strong since its release two years ago. Last year saw the release of a Linux version, The Old Gods expansion (which lets you start playing 200 years earlier in history), and The Sons of Abraham expansion.
Now, Paradox is readying another massive expansion, The Rajas of India. As the name implies, you will now be able to rewrite history as an Indian ruler; the entire Indian subcontinent is opened up. This doubles the landmass of the base game.
That's a lot of map -- map that you'll get as a free update to the game, but if you want to control an eastern territory and show those colonialists what's what, you'll need the expansion. And why wouldn't you want it?