FreeSpace (and not Freespace) is a late 90s space simulation in which the player, a space fighter pilot, must take part in the fight to repel the Shivan Incursion. The game is heavily reminiscent of the X-Wing series games, which were also space sims that were released in the 90s. It features basically the same mechanics, piloting style and general structure. The main differences lie in its atmosphere and story.
Although the game is pretty old, its moddability made it live very long, and the community still maintains the game engine for which the source code had been released in the early 2000. The fact that the developers released the level editor along with the source code for the game also contributed to make the game live far longer than expected.
The link to the Freespace custom missions messages page (for recording purposes)