Let's get a nice big list of awesome PC space sims that fans of FS 1 and 2 consider top notch. With that list, we can gain some voting momentum on what should be the next games for gog.com to offer.
I found my love of FS2 led to my love of Independence War 2: Edge of Chaos. It feels like Eve Online without the horrible MMO parts attached to it. Other than having great combat and an interesting story, the pirate mechanics made the game interesting and new while the music production had the best space-sim feel to it since... well... Freespace!
The obvious list also includes Freelancer and the X series, though I've never tried the latter because I've heard they're a bit deeper than I would enjoy.

Good idea for a thread. In my opinion you can never have enough Space Sims. Or rather there are so few that a list is a great idea. Points for you :)
Starlancer. Though people may have noticed that I pimp this game quite a lot :p
It's the prequel to Freelancer (or rather Freelancer is the sequel) but is a space combat sim rather than an open world game. It is fantastic. Right up there with Freespace and Freespace 2.
Youtube clip. This is mission 23. I chose this clip as it doesn't tell you any spoilers just shows gameplay. You can chose to fly 3rd person or cockpit. The cockpit is the way I play.
There has also been several forums mentions of the last level of Starlancer being "The Hardest Level Ever!"... Believe it.
Karl

No mention of the X-Wing series? Though I doubt GOG would be allowed to sell these (Lucasarts don't like other people selling their stuff I think.. or whoever owns Lucasarts these days, I lose track). But X-Wing was the first proper PC game I ever bought, and TIE Fighter was the second... superb games... also X-Wing Alliance wasn't bad either.

Don't forget the original Independence War and its add-on Defiance.

How could you guys forget Wing Commander? Though I'd love to see it on this site, it suffers from the same fate as Tie Fighter, except from EA.

I didn't suggest Wing Commander or X-Wing because I didn't want to attest for games I've never played. I think your game is more likely to gain votes if you give good reasons for voting for it, other than "Duh, it's X-Wing"

For me it would have to be Tie Fighter or maybe X-Wing as these were awesome in their day. I bet they look aweful now. Also, like someone stated earlier, Lucasarts probably wouldnt allow them to.
I dont know why Lucasarts stopped doing this series as they were way ahead of their time when first released. I am struggling to remember one brilliant game they have released this millenium.
I also played a very good game starring Clive Owen but the name escapes me, maybe Privateer 2 was its name

It was indeed :)
Karl

X-Wing was the game that made me buy a computer for the purpose of gaming. In the entire X-Wing series I felt like this is the closest I'll get to ever flying these craft. For space combat the series I thought was a little more in depth sim wise (energy transferances, shield arrangement, strategy and so forth) than that of Wing Commander. However the games were much more difficult to play. This series remains as my favourite of the genre.
Wing Commander, I felt was much more interesting as far as story and options go. Better conversation options, wingmate options, even deaths of characters if you allowed it to happen. The big thing I liked though was that unlike the aforementioned series if you failed a mission the game went on, like a real war, and your next set of missions was based on either your side pressing forward or retreating. You did have a few chances to right your course though and however things played out the ending reflected it.
I'm currently finishing Tie FIghter at this time and I've completed the Wing Commander series. I can't wait to play and finish Freespace as this will be my first experience with the series.

There is no greater space sim than TIE Fighter.
Closely followed by Wing Commander 3 and 4.
TIE Fighter (and the X-wing series) have the finest game engines I've ever come across for a space sim and are still the best example of feeling like you are in a Star Wars film. The engine in the Wing Commander games is not particularly special but the reason I love those games so much is for the stories.
TIE Fighter is also one of the most immersive and involving games I've ever come across. Difficult without being overwhelming and consistantly surprising in terms of narrative and gameplay. (PS It's my favourite game of all time.)

No to mention that the TIE pilots went from being Pants on Head Retarded in X-wing to actually being able to fly AND shoot at the same time and actually steal some of the mission kills away from you :)
Karl

It is absolutely correct to pimp Starlancer
I can't think of a space sim I've put more hours into than Starlancer, single-player and co-op alike

http://www.starshatter.com/
A game I have had recommended to me, but not actually played is 'Starshatter' (linky above!).
It's an indy game, and a bit more expensive than we're used to on GOG - but I thought I'd throw it in there.
I liked TIE Fighter and X-Wing (I still have the discs somewhere...) but my all time fave space sim was WIng Commander IV. Mainly because I found the story so compelling (especially being able to make choices in the cutscenes). Actually, I'd love to be able to play WCIV through again... *wistful sigh*
WCIII was OK, despite the uneasy feeling I got reading MAn-Kzin Wars years later.
Not actually played WCI, WCII or WC: Secret Missions. Are they worth tracking down?

Yeah, Wing Commander I and II are absolute classics, though obviously quite lacking in the graphics and presentation department now. Storywise they are just as good as III and IV.