i would like to recommend Freelancer's Discovery Mod :-
http://www.discoverygc.com/

There's quite an explosion of really good space sims now.. flowing on after Homeworld. I'm hoping these are what you guys mean by space sims :-

Sins of a Solar Empire
X3 The Threat (and its successors)
Nexus Jupiter Incident
Sword of the Stars
Allegiance
Haegomonia Legions of Iron (mmorpg)
Vendetta (mmorpg)
Yeah -- EVE Online
Jumpgate
Starport
Battlecruiser Online
(a numebr of others also exist)

oh and a very big Navy ship battle sim: Navy Field