Thanks to Gameogre, I have discovered Space Civilization, a free (as in free beer) MP game. A new round should start beginning of July 2008.
I have not clearly found out if it is a MP or MMO game, but the game is apparently scalable to the amount of players registering to play.
A slim client has to be downloaded to be able to play (a few megs only), but this GUI interface is really adding a lot to the game.
At the moment, you can play a tutorial of it, to get to practice before the big start. I have been told that the new round should start between the 4th and the 7th of July. The game is running on a ticker, which will be set to one hour to fasten your development, then two ticks, as the first skirmishes begin, to keep your social life unaffected.
I invite you to have a look at Space Civ here:
Space Civilizations
There is a live chat (not so live at the moment), a forum, of course. You'll find screenshots as well.
Enjoy.