Prime Directive (game)
Prime Directive is the role-playing game extension of the long-running Star Fleet Battles wargame series created by Task Force Games and Amarillo Design Bureau (aka ADB). Prime Directive allows you to create actual crewmembers for the ships of the SFB universe.

The original version ("1st Edition", usually referred to as "Prime Directive One" or "PD1" by fans) focused on on small, specially trained and equipped "Prime Teams" (essentially "elite" 5- or 6-member Away teams) who were sent on exceptionally dangerous missions. It featured its own unique RPG "engine" which many players found confusing and limited in its character development abilities. A sourcebook titled "The Federation" was published sometime later, and added rules that allowed for creation of standard starship personnel.

Two adventure modules were also published, entitled "Graduation Exercise" (which also came with a Gamemaster's Screen) and "Uprising". In addition, the first issue of a planned magazine-format supplement titled "Prime Adventures 1" was released, which included a preview of a planned Klingons supplement. However, Task Force closed shop before any further materiel were produced.

The more recent versions of the RPG--now under the auspices of ADB-- allow for more flexibility in character creation, as the game's base format has been licensed under three different gaming systems:

A "GURPS"-based version, licensed from Steve Jackson Games, centered on realistic gameplay using both its "Third Edition" (3E/Classic) and, later, "Fourth Edition" (4E) rules engines (it should be mentioned the 2005 edition actually used the Classic Edition TL scale rather then the 4e one);
A "d20 System"-based version, produced under the d20 open gaming license from Wizards of the Coast, which focuses on heroic class-based gameplay (like the company's Dungeons & Dragons RPG); and
A "d20 Modern System"-based version, with more up-to-date & realistic statistics than the original "d20" version.

Each version has its pros and cons in gameplay options, but all allow for more than just the basic "shoot-em-up" characterizations of the original version.

As of early 2014, the Core Rulebooks and supplementary volumes on the Federation, Klingons, and Romulans of the "Prime Directive" universe for the "GURPS 4E" and "d20 Modern" systems are available in both print and PDF editions through the Star Fleet Games website. (An additional Supplement for the "d20 Modern" version was also published.) Print editions of the Core Rulebook and the Klingon and Romulan volumes for the original "d20" system are also available (no original "d20" Federation volume was made). PDF editions of all the "PD1" material are also available.

Plans for versions based on two other systems have either been abandoned or permanently delayed:

A "d6 System"-based version, licensed from West End Games, focusing on more "cinematic" gameplay (similar to the original Star Wars RPG they produced in 1990s); possibly dropped when West End Games went out of business.
A "HERO System"-based version, licensed from HERO Games, which is a multi-genre system allowing for (presumably) extremely flexible gameplay. No word on the official site for any releases in this system.

Adventures and supplements
In addition to the Federation, Klingon, and Romulan supplements, one other supplementary volume and three mini-adventures have been published.

Away Team Log is a compilation of the RPG material published in nineteen issues of the Star Fleet Battles Captain's Log magazine. Most of the material is fiction and backgrounds based in the Prime Directive universe, so it is usable with any of the released editions of the game. The volume is available in print and PDF editions.
Three PDF-only "mini-adventure" modules – each original published as a free giveaway for the annual "Free RPG Day" event – are available for purchase and download:
Planet Aldo – an "away team" adventure (further details unavailable),
Starship Aldo – a space adventure for the crew of a multi-race salvage vessel, and
Dread Pirate Aldo – an adventure where a planetary survey team is kidnapped by the titular character in order to develop new weaponry for him.

Play Aids
As with many currently-published RPGs, the game's website offers a plethora of free downloadable Game Aids, including Character Sheets and other materiel (most in Adobe Acrobat "PDF" format).

External link
 StarFleetGames.com – official Star Fleet Games Prime Directive website
Role-playing games