X-Plane is essentially a rigorous flight simulator that simulates everything from helicopters and small civilian two-seaters to jet spy planes and the space shuttle.
If you're in the mood for a game where each event is reasonably well designed and integrated into a greater, meatier whole, then Infogrames' Xtreme Sports should prove satisfying.
X: Beyond the Frontier
If you miss the days of Elite and Privateer, then chances are you'll love X: Beyond the Frontier.
X-COM: First Alien Invasion
This is a scaled-down, greatly simplified version of the original.
Because of the lackluster space combat, Interceptor is the first X-COM game where the micromanagement and research portion is the more satisfying of the two halves.
X-Wing Collector's Series
These are classic games that belong on every computer gamer's shelf, no doubt about it.
X-Wing vs. TIE Fighter: Balance of Power
Balance of Power does an admirable job of addressing almost all the complaints gamers had with the original game.
This is almost the X-COM sequel that folks have been hoping for.
X-Men vs. Street Fighter
I like the Street Fighter series as much as the next guy, but this is taking it way too far.