Freespace 2 is currently free to claim on Good Old Games (GoG) for the next 42 hours from the time of posting.
The game continues on the story from Descent: FreeSpace, once again thrusting the player into the role of a pilot fighting against the mysterious aliens, the Shivans. While defending the human race and its alien Vasudan allies, the player also gets involved in putting down a rebellion of those elements of Vasudan (Hammer of Light) and Human (Neo Terran Front) forces which don't want to cooperate with one another.
Freespace 2 also includes the FRED2 editor available in the installation directory of the game.
Link: Freespace 2 on GoG