If you're a big fan of the classic Civilization game then Freeciv might be for you.
Freeciv is a turn-based strategy game that works in real time and as the name suggests, is free to play. Your job is to manage the economic and military might of an entire civilization. Freeciv is clearly inspired by Civilization and although you can't expect it to deliver everything that the original did, it's an impressive effort for a free game.
Your main goal in Freeciv is to survive as long as possible starting with just a couple of plots of land, a settler and an explorer. Your aim is to to conquer as much land as possible, defeat as many nations as possible and ultimately reach the year 2000 AD. The fun thing is you can play alone or in multiplayer mode via LAN or net with up to thirty players.
Once you've made your moves, you need to keep an eye on what your opponents are doing but this isn't easy when so many variables change such as your economy, political situation, the weather etc.
The end result is that Freeciv is an absorbing game which will appeal to anyone who liked Civilization.