I've been all over the place with space sims lately this is a good topic.
I wouldn't say that Freelancer and Freespace are that similar. Freespace is pure action (at it's best). Freelancer has combat, but it's not the only thing since it adds trade.
If you like Freespace, you'd probably like Dark Horizon, which is very similar.
If you like Freelancer, you'd probably like Darkstar 1, which is actually better IMO, although it mimics it closely. Better than that is IW2, as mentioned above.
Overall, my favorite is Independance War 2. Cockpit, trade, a lot of action, physics give the impression of realism but not over the top. Doesn't handle so perfectly like Freespace though---in a good way that is.
Spaceforce: Rogue Universe is a shooter but also has some trade. If only the scripting/voicework were better! Storyline/voicework is a little too anime-like which tries to screw up an otherwise really good game. This one has likable physics too.
Parkan 2...I recently got this and don't have much time in it. However, it's cool in that it adds ship boarding and changes to an FPS to clear the ship. I haven't gotten to it yet, but it also has planet-side ground work as an FPS. Should be a lot of fun, but I've got too many going as it is to get further into it right now.
So far, IW2 is my go-to game. Everything about it is well done. Only drawback is that space is a little too stylized with a lack of black space---it's all colored nebulas etc for a background.
X2 I like. Moreso than X3 in fact. X3 has endless menus and never gives the impression of getting off the ship. A little too deep though, these take a while to get into, and if you play more than one game at a time, it's hard to stick with.
Next in line for the search for the best space sim: the Evochron games.
