If you like civ, then chances are you'll probably love Battle for Wesnoth. I've never seen a free game to be that well done - it easily beats some of the commercial products out there. I wasted most of my weekend playing it:
http://www.youtube.com/watch?v=EIt69vznegs
http://www.youtube.com/watch?v=EIt69vznegs
The Battle for Wesnoth is a Free, open-source, turn-based strategy game in a fantasy setting for Windows, Linux, Mac, AmigaOS, BeOS, FreeBSD, OS/2 and several others. Get it at http://www.wesnoth.org.