The Hashshashin

The Hashshashin were an Islamic community in The Middle East around the time of The Crusades, who both named and codified the Assassin archetype, though only few know to what extent their image has been Flanderized since then.

Originally an esoteric Islamic Cult—usually classified as a radical offshoot of the Isma'ili sect of Shi'a Islam—founded by Hassan-i Sabbah, it developed into a society of assassins operating from the mountain fortress Alamut and terrorizing the region's rulers with their attacks. Despite their fearsome reputation, the Hashshashin were said to be quite friendly towards common folk, since their killings were actually carefully targeted and planned, rather than random acts of violence. Their name stems from their use of hashish—allegedly, the recruits were drugged, led to a paradisiacal garden, full of lush greenery and beautiful women, and then told that only the "Old Man of the Mountain" had the means to let them return, which resulted in their fanatical devotion. Their downfall came when one of their strikes reached the Mongols, who razed Alamut during their conquest of Persia.

In fiction, The Knights Templar are often positioned as the Rivals and Arch Nemesis of the Hashshashin (and the two are often found alongside each other in Conspiracy Kitchen Sinks); this can be quite frustrating to people who know the region's history, since Crusaders—though not necessarily the Templars themselves—were often allies, since they shared a common enemy (i.e. the Sunni Turkish and to a lesser extent Kurdish and Arab lords who ruled the region, including Zengi and Saladin). Compare Ninjas, the Far Eastern counterpart.

References to the Hashshashin in fiction

Comics

Jonah Hex fights a Hashshashin brought to the Wild West as part of a Carnival of Killers during the "Six Gun War" storyline.
In one story of the Conan the Barbarian comic book, he ran into a cult that was an obvious Expy of the Assassins, including the garden of Paradise.

Film

Part of the antagonist force in the Prince of Persia the Sands of Time movie. Curiously, Alamut is featured in the film, but it is not their main hideout.

Literature

The Destroyer series of novels, being about assassins with a ancient heritage, naturally mention the Hashshashin.
In Foucault's Pendulum, the Templars learned from the Hashshashin during the Crusades to discover the secret of harnessing the power of telluric currents.
- Incidentally, it is one of the few works featuring both Templars and Hashshashin, where they are not said to have been at each others' throats. Props to Umberto Eco who Did the Research.
Not sure about the context, but there were a few Flashbacks featuring the Hashshashin in The Illuminatus Trilogy.
The Dragon from Angels & Demons invokes this reference by calling himself "Hassassin" (in addition to being of Arabic descent).
Parodied in Discworld with the hashishim, who are a bit too heavy on the hashish bit..
Vladimir Bartol's novel Alamut embraces most of the historical legends of the hashshashin, and was the main inspiration for Assassin's Creed.
The Mongols go up against the Hashashin in the third Conqueror book, aghast by the group's cowardly and bloodthirsty ways. Genghis Khan personally kills the Old Man.
Mack Bolan, The Executioner, fought a revitalized cult in Assassin's Code. The Assassins in this one recruited various mercenaries including Ninjas, Thugee, and a rogue Mossad agent.

Live Action Television

They appeared in both Robin of Sherwood as a dangerous sect and in the BBC's Robin Hood as an Amazon Brigade.

Music

Hawkwind's song "Hasan i Sahba" links the Hashshashin to modern Islamic fundamentalists.
The song "Garden of Light" by Isis, a post-metal band, refers subtly to the the Hashashin recruitment procedure.
"Wine of Aluqah" by Therion. "Know that nothing's true and that everything is permitted,/So read the Old Man of the Mountain in his Book of Lies".
There's the song "Haschichins" by French punk band Pachuco Cadaver.

Tabletop Games

The Dungeons & Dragons Al-Qadim setting has numerous "Holy Slayer" groups inspired by the Hashshashin. Adventure ALQ2 Assassin Mountain has the Everlasting, a cult that live in a mountain that are clearly based on them.
Vampire: The Masquerade had the Assamites, which were basically what happens when an ancient vampire decides to take over the Hashshashin from within and bend them to his purpose. The clan still works out of Alamut and hires themselves out as assassins.
The Legend of the Five Rings CCG spin-off Legend Of The Burning Sands had a group called the Assassins in it, who excelled at killing in duels. While they were led by the "Old Man of the Mountain", most of the other characters in it were women.
In the wargame Infinity, Hashashin are special troops that can be fielded by the Haqqislam faction, divided into four distinct types: snipers/poisoners, close-combat specialists, stealth experts who can pretend to be an enemy unit, and explosives specialists.

Video Games

The Gothic series features a blatant expy of them called the Hashishin, whose weapons are assassin style blades (and often coated with poison). Of course, they are also blatant stereotypes of clasic Arabians in that they are very into economics as Serious Business, not to mention a rather cliche habit of giving people Overly Long Nick Names.
The Assassin's Creed series posits that the Hashshashin order was simply one part of the Crusades-era incarnation of a secret society of professional killers who have existed throughout human history, whose primary goal as been to use their skills and methods to preserve peace and protect the innocent from those who would abuse power. Their primary enemy are The Knights Templar, another secret society with similar reach, but with wildly different goals. It turns out that the Assassins are the descendants of Adam and Eve, who were a pair of humans who were Half Human Hybrids created by the First Civilization and who were immune to the effects of the Pieces of Eden, allowing them to lead a rebellion against their masters.
Bad Guys in Lionheart, the bugged, prematurely released fantasy version of Fallout.
Lost Souls MUD features the Nizari, a hybrid of the Hashashin and the Thuggee. This Is Wrong on So Many Levels.
In the Nasuverse, Hassan-i Sabbah is a Legacy Character. Whoever is the master of Alamut must take the name and appearance of Hassan, and abandon their own identity. The Fate series' Assassin-class Servants are generally required to be selected from the Hassan-i Sabbah; however, in Fate/stay night, a violation of the rules results in Sasaki Kojiro being summoned as an Assassin. This is corrected in the Heaven's Feel route when Zouken summons a proper Hassan-i Sabbah as "True Assassin", which kills the original Assassin in the process. True Assassin proves to lack the limitations of the improperly-summoned original Assassin and be far more effective and dangerous.
Broken Sword featured a mysterious murderer who turned out to be a Hashshashin trying to foil the plot of the Knights Templar villains.
In Medieval II: Total War, Islamic factions can build Hashshashin Guilds in settlements where large numbers of spies and assassins are being recruited. Doing so improves the effectiveness of spies and assassins recruited there subsequently, as well as allowing the faction to produce specialized Hashshashin infanty units, which serve as small, elite heavy infantry capable of ambushing on the battlefield.
The Alik'r Warriors from The Elder Scrolls V: Skyrim seem to be based on the Hashshashin, in that they're an organisation of assassins who come from a Middle Eastern-like setting - albeit one based more on Morocco than Arabia.