I was surprised that this one isn't up here yet. Great 4x game, and a pretty dedicated mod community. The game itself is a few years old now but there are quite a few games in the series. I've heard people compare it to Eve but I have never played Eve so I wouldn't know. The game itself is sandbox, but there are different story lines you can take on. Game play wise it compares to something like Privateer (really old space sim) or Freelancer, lots of trading and fighting (if you want it, I prefer trading). The big difference is that you can build your own space stations and pretty much grow your own galactic empire. The graphics are great, and the universe is pretty big.
Anyways, I guess this isn't much of a review and more of a suggestion. It is definitely one that is under the radar and I was more than surprised when I had found it, and then wondered why oh why hadn't I found this game sooner. It's a pretty complex game to begin with, and adding the fact that you can write scripts for it and mod it pretty much makes it as complex as you want.
For all I know everyone in this community is well versed in the 'X' universe, but for anyone that has never heard of it and wants a complex space sim that includes trading, building, and fighting I highly, highly recommend it.