In Terry Pratchett's Discworld series of fantasy novels, there are almost 300 Guilds in the city of Ankh-Morpork. Nanny Ogg's Cookbook, The Thieves' Guild Diary and the Death's Domain map all quote Guild publications. Guilds known include the following.
(mentioned in Jingo)
(mentioned in The Truth)
(seen in 'Discworld Noir', a computer game based around a detective on the Discworld.)
(mentioned in The Truth)
(mentioned in The Last Hero and Thud!)
The Assassins' Guild follows strict professional guidelines, with members behaving at all times in a gentlemanly manner. The guild is one of the richest in the city, and runs its own school, like the thieves. Unlike the thieves, however, the school is one of the most exclusive, and sons of the nobility - and now daughters - attend, though some are "excused stabbing". The school also educates scholarship boys - frequently from the Shades, and other poor areas of the city, these are the kind of people who are good at killing, Mr Teatime and Inigo Skinner being notable scholarship students in the guild school. To date, there are two people in Ankh-Morpork on whom the Guild will no longer accept contracts - Lord Vetinari and Samuel Vimes, Commander of the Watch. Vetinari is 'off the books' because he's both too important to the stability of the city and infinitely preferable to any potential successor, while Vimes simply has a reputation among the Guild for being more dangerous than the assassins that are sent after him.
(mentioned in Guards! Guards! and Feet of Clay)
(mentioned in Lords and Ladies)
(mentioned in Lords and Ladies and "Medical Notes" in Once More* With Footnotes)