FreeCol is an open source clone of Sid Meier's excellent turn-based strategy game, Colonization.
The story starts in 1492. You lead a tiny band of settlers who've just arrived in America, with just one goal: to make the country your own. But these are challenging times. You'll need to earn money by producing goods and sending them back to Europe. There are cities to build and manage, and new technologies to learn. And there's another complication: you're not alone.
Other European powers are building colonies of their own, expanding their borders and becoming all too powerful. War can't be too far away.
And then there are the native Americans. Will you become their friends, learn new skills and trade with them? Or attack their villages and take what they have by force? It's your call.
The ultimate objective is to become powerful enough to declare your independence from Europe. The King won't like it, and an attack force will be sent, but if you've prepared well, and are strong enough to defeat it, then congratulations are due - you've won the game. And now you've mastered the basics, you can try the multiplayer features, and pit your skills against other players.