Weekend Extra

This week's older game will be FreeSpace 1.

FreeSpace was made by the same team that created Descent. Interestingly enough, they are vastly different styles of games (though they are in the same genre.)
FreeSpace is a mission-based space flight sim. The basic plot is that Humans and Vasudans are at war when the Shivans show up and start indiscriminately blasting people out of space. The Humans and Vasudans (of course) join forces to fight the greater foe. While this plot is not exactly unique, and many space sims have used it (*cough*WC:Prophecy*cough*) it works pretty well. This is because they bothered to include factions within each side that are not happy with allying with their former enemies.
Combat in FreeSpace was well done, a semi-airplane set of physics makes the dogfighting both easier and more enjoyable than strict Newtonian physics would have been. The weapons are a real gem in this though. You begin with just simple energy weapons that deal no shield damage, and unguided low-yield missiles. As the game progresses, technological breakthroughs are made by the combined scientists of the Humans and Vasudans. This means that every few missions, new weapons show up. But you don't just get these weapons, oh no, often you have to test them, or fly the prototypes from point A to point B. And what happens there matters, you can get them earlier by playing well.
Ships are handled in basically the same way weapons are, new ones as they are developed. There is a great variety in ship types; interceptors, space-superiority, light bombers, heavy bombers, etc.
I should talk about the customization abilities next. Again, when the game begins, you are just a wingman in some squad. As the game progresses, you eventually make your way to squad leader, and then eventually leader of 4 squads. At the beginning of a mission, any ships you are allowed to control are given a recommended loadout, the fun is in that you don't have to keep that at all. The mission might say to use a heavy bomber wing, 2 interceptor wings, and a light bomber wing. But you can change it so that each squad consists of 2 bombers and 2 interceptors, or really anything you want. This adds a level of customizability to the setup of missions, not just the mission itself. Also, the weapons, ammo, and ships available are actually kept track of. So if you take those three brand new space-superiority fighters out on the mission, and get them all killed, good luck trying to field any until the next supply run. This adds a whole, almost hidden, level of resource management across missions.
The last is really the best. Non-linear mission structure. Or, well, something close to it. Essentially, your performance on missions (and what actually gets done, yes, often you are allowed to fail objectives) determines later missions. Suppose you are given a secondary objective of destroying a specific cruiser in a mission, but it warps out before you can finish it off, it WILL show up in some later mission to cause you strife.

Sure, the graphics are old, but otherwise, it ranks as the most entertaining space sim I have played. Ranked right up there with X-wing and TIE Fighter as one of the best. Sadly, it has not been majorly influential on the genre, and people still get excited over such old ideas as non-linear mission structure.