SMAC

The old game of Civilization, if you've never played it, had a top-down view, with fairly minimalist graphics. You built cities, made 'settlers' (the SMAC version of a settler is a former) to build more cities and to irrigate and farm different plots of land. The technology expanded only at your control, so you could have modern tanks riding across the ocean on 17th century troop transports. Despite, or perhaps because of, its few shortcomings, the game flourished, and was quickly adopted as one of the best strategy/war games of its time.

In Civilization, you could be one of the ancient civilizations (Greeks, Aztecs, etc.). In SMAC, however, you take control of one of seven 'factions'.
The story is that a colony ship was damaged just before landing on the first viable planet it had found. Seven 'colony pods' were released in a last ditch effort to save the mission. All seven pods made it to the planet, which was dubbed Planetfall. Upon landing, the pods each chose a leader from those upon their pod, and so the factions were formed. Each faction has its own strengths and weaknesses, such as The University of Planet, which researches 'techs' faster.
One feature I'd like to bring up before closing is the unit creator. Some of the new techs you'll research will, of course, have military value, or will be military in nature. When you learn these new technologies, you have the option of creating new and better military units. You can either let the computer do it for you, or you can make them yourself. There are well over 100,000 combinations you can use, but many of them would be useless. A former with missile-launching capabilities would of little use, as they're non-combat units and can't attack.

For more information, you can visit alphacentauri.com, or firaxis.com. For the Macintosh version, however, you'd be better of visiting aspyr.com.