Independence War 2: Edge of Chaos
Not as complex or as difficult as x3, but has a HUGE Universe, weapons, and great graphics even by today's standards. The story is decent, but a lot of Free Roaming as well. In fact, I like it much better, than Freelancer, as it uses Newtonian Physics, and the ships actually feel different from each other. A fantastic game if you liked Freelancer and much less arcade like (more simulation).
For space shooter sims: Freespace, Freespace Silent Threat, and Freespace 2 (this is free with great graphical enhancements and mods).