Hi, I'm going to post here some gaming alternative to Freeciv, with some comment. Each.
1. The Kerbal Space Program.
If you like open worlds, personal challenge as well as careful planning and creative strategy, you can't miss KSP.
IT is not a wargame, and even less a 4x game. It is just a incredibly realistic phisical simulation of a Space Program of Planet Kerbal, home of some little humanoids you have to help reaching their Moon "Mun" and other planets. You'll learn TONS of things about real-life space technology, physics of orbit and space traveling, and and and. All with "common" technology of today.
The Demo is perfect to learn some basics and decide if you're ready for the Program.
Be careful, though: to reach the Mun without destroying your space ship is a hard task to accomplish, while coming back to Kerbal, well, it is almost a dream in the 1st weeks of simulation. Other planets? The amount of planning and preparation of such a Mission is so incredibly complex that you'll need months of simulation to reach them, let alone building bases there.
I gave up after reaching the Mun and succesfully left its surface even if my little Kerbalians didn't get it safe home. Let me know your own space exploration stories!