Talos Cult
The Talos Cult is a group of worshippers of the great Tiber Septim, founder of the Empire. In order to join, new recruits must swear the following oath: That we shall die to put a strong man back on the throne of Tamriel. They have many followers in the Legions, conspiring to assassinate whoever the current emperor may be and replace him, so to prevent the Empire's decline.
 Temple of Kynareth
"Our Lady is the Goddess Kynareth whose sphere is the Heavens itself. She is sometimes called the Goddess of Air, the Pure One. It was She who gave the Lords Mail to the Warrior Lord Morihaus to combat the unclean Daedric spirits. By the glory of our Lady, we have become the greatest physicians in Tamriel -- our ability to cure magical and mundane diseases is truly unrivalled. Other temples cure only their own priests, but we are philanthropists. Of course, the Goddess shows special favor on Her own. We do take in new initiates, provided they show devotion to the Goddess and have the skill to make their devotion worthwhile." -- An unknown Kynareth follower
 Temple of Stendarr
"Our God is the God of Mercy, Stendarr. Some mock our compassion, calling us sentimental fools and tenderhearts. But the truth is that everyone, no matter their strength, crawls to us when Sai, the God of Luck deserts them as he is prone to do. Our gift to the traveller is a simple one -- we heal those who are in pain. For more complicated ailments, diseases and poisonings, we direct you to the Temple of Kynareth. With due respect, for they are the finest apothecaries and we, the finest chirurgeons. If one possesses the desire and aptitude, one may join the Temple. Though we desire to help all Tamrielans, we only possess the means to train and assist a small handful. Sometimes, it is best to begin one's compassion with oneself." -- An unknown follower of Stendarr
 Temple Saints
The Temple Saints are the most honored ancestors of the Dunmer, canonized by the Tribunal Temple for deeds they have performed during their lifetime. They are celebrated as models of virtue and piety, often invoked for aid and protection. After the fall of the Tribunal, the three Tribunes were re-appointed as saints by the New Temple.
The Thalmor was an Altmeri government established in Valenwood to serve as their representative in the Aldmeri Dominion (circa 2E 830), a union of the Bosmer and Altmer provinces. They claimed they had the authority to create this government on behalf of a local Bosmer claimant, Camoran Anamexes. As Valenwood possessed no true government prior, the Thalmor purported to maintain order in the region to uplift their Bosmer brethren. The formation of the Thalmor was not an altruistic gesture, however: the Altmer sought to control Valenwood because the region, along with southern Hammerfell, harbored many pirate groups which threatened the shore of the Summerset Isles. The Altmer had become fed up with their long history of pirate attacks, and sought to secure the shores of Summerset by leaving those pirates no place from which to attack them. The Thalmor was dissolved a short time later when Tiber Septim brought Valenwood into his Empire using the overwhelming power of the Numidium.
Centuries later, a new incarnation of the Thalmor arose in the Summerset Isles, reportedly claiming to have saved the Altmeri people from the Oblivion Crisis. In 4E 22, the Thalmor took control of the Summerset Isles and renamed them Alinor. Valenwood was brought under the Thalmor's control in 4E 29, recreating the Aldmeri Dominion, and later, what was once the Elsweyr Confederacy reverted back to the ancient kingdoms which had formed it, now acting as client states of the Thalmor. It is said that of all of his enemies, Titus Mede never managed to defeat the Thalmor, and it would be Titus Mede II who would pay the price. When Titus II inherited the Imperial throne, the Thalmor instigated the Great War with him by delivering a series of ultimatums to the Imperial City, then presenting the head of every Blades agent in the Aldmeri Dominion when he refused their demands. After four years of bloody war, the Emperor was forced to sign the White-Gold Concordat, essentially giving in to their demands. One of the terms was the outlawing of the worship of Talos, and the right of the Thalmor to enforce that ban within the Empire. Another term of the Concordat was that the Empire relinquish large tracts of land in southern Hammerfell to the Dominion. Even though the Empire acquiesced, the Redguards of Hammerfell fought on and eventually repulsed the Thalmor from their territory, leading opponents to believe that the Concordat was a mistake, and Titus Mede II overestimated the Thalmor's strength when he signed it.
Nevertheless, the Thalmor have the legal authority to root out and persecute Talos worshippers. They carry out campaigns to do so from their embassies in the various provinces, which also allows them to gather extensive information on the Empire's internal affairs. It's widely known that they see men as inferior and themselves as the rightful rulers of Tamriel, and it's believed that it is only a matter of time before they seek to act on that belief again.
 Thieves Guild
The Thieves Guild is dedicated to the gathering and training of those who are stealthy and shadowy in nature. Although criminal by its very definition, for untold years, local authorities in places throughout Tamriel have tolerated the existence of the guild for its role as "crime regulator," as it does not tolerate competition or egregious conduct from its members (not to mention the personal financial benefits for authorities who play nice). It is usually considered to be a distinctly Imperial entity, though of course other organizations of the Empire do not officially condone their actions.
Like any trade guild, the Thieves Guild is an organization of professionals, except that in this case the professionals are burglars, robbers, pickpockets, smugglers, and other enterprising operators. They typically don't have public guild halls, but sometimes have safe-houses, and members tend to gather at a single location in large towns, such as at a cornerclub, inn, or tavern. Typically, members are bound not to rob each other, kill anyone while thieving, or rob the poor. The Thieves Guild usually has the resources to bribe officials, establish a black market of stolen goods, and maintain a network of informants.
 Tribunal Temple
The Tribunal Temple was the native religion of the Dunmer of Morrowind. They worshipped Almalexia, Sotha Sil, and Vivec, known together as the Tribunal or Almsivi. Most people usually just call it "the Temple". They accept outlanders as members, but most of the few outlanders that do join, join only for the services provided. On Vvardenfell, the Temple District included the city of Vivec, the fortress of Ghostgate, and all sacred and profane sites (including those Blighted areas inside the Ghostfence) and all unsettled and wilderness areas, as the Treaty of the Armistice with the Empire maintained the island as a Temple preserve. In practice, this district included all parts of Vvardenfell not claimed as Redoran, Hlaalu, or Telvanni Districts.
The Temple maintained less than favorable relations with most of the Imperial guilds of Morrowind. However, it maintained strong relations with the pious House Indoril and House Redoran, and was friendly with House Dres and House Hlaalu. When the Tribunal fell apart, the Tribunal Temple underwent a reformation, becoming the New Temple.
 Twin Lamps
The Twin Lamps is the secret, underground slave recovery and rescue guild of Morrowind. The group helps free slaves, primarily Argonians and Khajiiti and transport them out of Morrowind to safety. They use the Argonian Mission in Ebonheart as a base of operations. Their head in Morrowind is Ilmeni Dren, the daughter of Duke Vedam Dren. They commonly use the phrase: "Have you seen the Twin Lamps?" to recognize fellow members, the answer required being: "They light the way to freedom."