While a couple of years old, this game deserves to be highlighted since apparently there is no thread for it.
It well umm...I'm hesitant to call it a "freebee" because it's an AAA-quality game that rivals anything you could pay money for. Lasers! Explosions! Also a huge amount of voice acting, some of which is done by actual professionals. The 8GB file size is justified.
With the current crop of space games still mostly in development, I'm sure some of you are trying to find something. Here you go.