Turn-based Asynchronous Game Recommendations?

I have a few friends that live in various parts of the country. We're all pretty busy with work and families. We'd like to play a game together but it's hard to find the time.

Are there turn-based games out there that can be played asynchronously with turns maybe having to wait days before being acted upon? If so, what are some of the best ones? Ideally, we'd like a sci-fi setting, but pretty open if the gameplay mechanics are compelling and if there's a decent story.

We all have Macs and Xboxes, so one of those platforms would be a requirement.