I'm not sure its FOSS, but its available in repositories so there is some degree of it being FOSS.

I really like Endless Sky; its basically a rebuild of Escape Velocity, and I think it's a great game.

It's a 2D space opera, you start out as a small shuttler and work your way up to massive fleets.

I don't remember much about Escape Velocity, but Endless Sky has a pretty decent story as well.