Deviate: I can't believe it took this long for someone to mention Tachyon: The Fringe. I still got the CD for it, but it won't run on my Vista machine. *sad*

darkenedroom: I have to agree with most of these posts and throw in my own 2c:
Starlancer: I had a copy of this but lent it away and never saw it again, but from what I remember it was a graphically impressive and a high degree of immersion. Both challenging and fun. I would definitely recommend this game for inclusion.
X-Wing Collectors Edition: I still have this and yes it is hard (and frustrating in places) but there's nothing like the sense of accomplishment of beating the mission you've been stuck on for days. Great music added to the experience. Another excellent game that should find a home here.
I-War: Played the original but not the expansion. For me this game was just stunning, and there was a real sense of flying a 'big ship' corvette. This coupled with the Newtonian physics made it my favourite space simulation for quite some time, until I-War 2. I never played the Defiance expansion, but I think I'd buy this again if the expansion was bundled.
I-War 2 - Edge of Chaos: A fantastic sequel to the original I-War, builds on what came before with a interesting and engaging plot. The Newtonian physics is simplified a little reducing the learning curve of the original. My only criticism is that it loses the big ship feeling of the original (I think). Still one of fav space simulations.
Privateer 2: Recently sold this on eBay. Found this engaging but also frustrating in places, success on some missions depends more on luck than skill due to the random pirate encounters through space, but don't let that put you off too much, there still a lot of fun to be had.
Wing Commander 3 & 4: Sold these on eBay recently as well. these were fun but I never found them as much fun as some of the others listed here (which makes me think I missed something, as many others have raved about them). IIRC they are fairly large in size (and hence may be an issue to download) with WC3 taking 3/4 CD's and I think WC4 may have taking up as many as 6.
Hardwar: Played through the single player plot on this a few times. Great game atmosphere great game mechanics good plot and a still thriving fan community with patches and enhancements released regularly that have added a great deal to an originally great game.
Tachyon: The fringe: Voice acting by Bruce Campbell is great. The ability to play either side of the conflict after a plot branch adds to the re-playability value. More gamey than simulation. Fun but there are better, I think I would rank this alongside Privateer 2.
Freelancer: Only ever played the demo, and I did want to buy this, but I just never got round to it. Would probably purchase this if it appeared on GoG.

katakis: I just have to ask this question: does any of you chaps know EPIC?

custardcream: like your user name, it was a commodore amiga game if i recall.
flight sim in space wasnt it??

Barelyhomosapien: I'll echo what quite a few people have said and bring up Starlancer, one of my top 5 games I reckon and mostly for the storyline, characters and high production values. Alittle easy and simplified to play, though not arguably a bad thing as it was still alot of fun.
If I remember rightly wasn't it a microprose game? If so that could lead to other fun places like the Mechwarrior/Commander game.
I think the game that first ignited my love for space sims was wing commander, like alot of people. Though I think unlike alot of you it was wing commander on the SNES! Can't remember if I ever managed to finish that game.
A question for the Starlancer fans! Was I the only one that found Freelancer immensely disappointing? I remember how disappointed I felt when I found out it didn't even have joystick support, it made me wonder what sort of game they were actually making.
And I know it's not exactly the same sort of game, but Star Trek: Bridge Commander has to be the best trek game made. One of the very very few, decent, big ship sims and I'd highly recommend it!

Guibs: Ah most of you guys know their stuff.
Space sims have been for many years my favorite genre.
It all started with tie-Fighter like many of you. I own all x-wing/Tie-Fighter games (including Alliance).
My fetish series though as always been Wing Commander. I played them all to completion and was simpy delited. The by move was obviously with Wing Commander III and IV. The story was excellent (Story is the one of the biggest thing that wins me over in a game) and the Actors in there, including Mr. McDowald as Admiral Tolwin was memorable.
After Wing Commander, I was at a lost until the guys from former Wing Commander game created a new studio (Digital Anvil). StarLancer came out it was a great game. Then Came FreeLancer, who wasn't has great as I thought it would be.
the last space sim I bought was X3 Reunion. I was desapointing as the gameplay is closer to EVE than the classing Action space sim. The interface is horrible and the lurning curve is amazingly high. you almost need a course to play that game.
For some reason, I totally missed the Freespace series. I'm seriously considering getting the second one and give it a try just for good old days sakes. I wish a high profile, great space sim games was in developpement and coming soon. The latest attempted were less that stellar compared to what the X-wing/Wing Commander series were at.