Harvest resources, expand your operation, and survive waves of enemies, all in real time. It doesn't sound like anything special, but The Space Game: Missions is mesmerizing. You'll build beautiful, sprawling, weblike structures, and then watch as they tear apart enemy fleets and send repair bots swirling through their hubs and spokes. It's intense, and at times quite challenging.
TSG: Missions has a solid tutorial, a fantastic interface, multiple difficulty levels, and a ton of optional missions. If you're going to add another casual game to the already crowded strategic defense genre, this is definitely the way to do it.