Turbo Trek is a Star Trek inspired MS-DOS game by JDG Simulations from the days when Windows 3.1 was still warm. It's a turn-based starship combat game with easy-to-use ship and scenario creators. From what I can gather, I am far from the only one who found the game eminently entertaining.
Turbo Trek and FreeSpace show that giving users content creation tools is a good way of extending replayability, even for old games.
The sum-total of my Turbo Trek knowledge. It was written almost completely from memory when I wrote it.