Prime Directive is a role-playing game set in the Star Trek-derived Star Fleet Universe. The game has gone through two different incarnations.
The original release of Prime Directive was published by Task Force Games in 1993. Using an in-house design by Mark Costello and Timothy D. Olsen, it used a D6-style system similar to Shadowrun and Star Wars. PD’s most popular aspects were a multi-tiered task resolution system, where characters could achieve 'Complete', 'Moderate', 'Minimal', 'Failure' or 'Botch' levels of success with their actions, and the Character Reputation and Background system that allowed characters to “spontaneously” develop skills, abilities, and prior associations appropriate to the mission at hand. Both of these aspects provided a sense of Television Series verisimilitude that many found to be appropriate to the game’s Trek origins. The greatest complaint was that character selection was limited to members of a Prime Team or a similar group, not allowing players to be captains or other important positions, although later supplements did provide this ability. Also, since the rules were unique to this game, and only vaguely similar to other systems, many players were not willing to take the time and effort required to learn them, although the same could be said of many RPGs of that time.
The published books for the game included the core rulebook, a Federation sourcebook. and a few adventure books.
GURPS Prime Directive
After the folding of Task Force Games and the cancellation of the original Prime Directive line, Amarillo Design Bureau, Inc., the new publishers of Star Fleet Universe games, were left with the question of what to do with PD. They were primarily a wargame company, and the poor sales of the supplements were cause for a reexamination. Eventually, they approached Steve Jackson Games to utilize the Powered by GURPS format.
D20 Prime Directive and D20M Prime Directive
At this writing, the core rulebook, D20 Klingons, D20 Romulans (no D20M as of Aug 2011), and D20 Federation are available for this system. Supplements for the Gorns, Tholians and other empires/races are in various writing stages.
Source: Wikipedia, "Prime Directive (role-playing game)", available under the CC-BY-SA License.