Adventure Guild

This is an organization, and we use the term loosely, that enrolls adventurers and gives them access to jobs. Jobs generally range from "find my cat" to "destroy Omega Volcano Satan", and are posted by random locals who can't, apparently, do anything for themselves.

Jobs generally are posted on a bulletin board (or encrypted dataserver or cork slab, depending on the setting), where certified adventurers sign up for them. "Certified Adventurers" is basically a euphemism for spiky haired obsessives, teen girls who will almost certainly turn out to be princesses, and other protagonist types.

If Craigslist had a section labelled "Jobs wanted: Medieval Commando Squads", it would be one of these.

Mainly a video game trope, but also shows up in Anime, especially when the setting is a Role Playing Game Verse. Mainly shows up in Japanese works, because the Japanese seem to feel that even killing people and taking their stuff should be done in a structured, social context.

A subtrope of We Help the Helpless, but there's something more specific here. It has to do with the concept of a medieval guild adapted to serve the needs of a video game.

Competition or open war between rival guilds is a common plot point.

One more thing: Although the main characters are described as a guild, they usually don't have a common skill set. Fighters, mages, and thieves (and others) can all work for the same guild, but won't learn skills from each other. After all, that would make them similar, and What Measure Is a Non Unique? (There are occasionally organizations that cater to these types, but then we get into politics.)

See also Weird Trade Union, Murder Inc, Thieves Guild. Compare The Order, which has a more rigid structure and better-defined purpose.

Video Games

 * Monster Hunter
 * Final Fantasy XII
 * Resonance of Fate
 * Final Fantasy Tactics Advance
 * Arc the Lad
 * The latter Mega Man Battle Network games have bulletin boards full of Side Quests to do. Which ones can be done is typically limited by a ranking system.
 * Avernum has a bulletin board in every town that supplies you with jobs and quests.
 * The old Eamon computer game had the Guild of Free Adventurers, the place where you always started your adventures and where you returned when you finished. It had a number of functions you could access while there. May be the Trope Maker
 * Your party in the Etrian Odyssey games is one of these.
 * This is the primary mechanism to get missions in Wing Commander Privateer.
 * Ditto generic missions in Escape Velocity.
 * Pokémon Mystery Dungeon: Explorers of Time/Darkness/Sky is all over this - they take 90% of your pay though.
 * The Elder Scrolls games have something like this, but those are more function-specific and function more like actual medieval guilds than the odd job clearinghouses that epitomize this trope.
 * The Adventurer's guild in the Quest for Glory series, although they typically only granted you the low level stuff, big things had to be done by convincing people you were the one to do them.
 * The Trouble Center in Paper Mario the Thousand Year Door
 * The PSP game Legend of Heroes features one of these.
 * Final Fantasy XIV
 * Agarest Senki 2
 * Quest Cards in the first Fable game.
 * Star Wolves
 * Rune Factory 2 and Rune Factory 3 have a bulletin board where townspeople post quests
 * Phantasy Star had a Hunter's Guild starting from Phantasy Star IV and onward . . . As of Phantasy Star Universe they were renamed Guardians but essentially do the same killing of monsters and handling of odd jobs.
 * In Solatorobo, the adventurers are called Hunters, but there are many guilds, the largest of which are the Kuvasz.

Anime and Manga

 * Fairy Tail every mage's guild.
 * Soul Eater has a very videogame-esque job board at the school, complete with estimated number of souls the students will receive on completion of the mission.

Tabletop Games

 * Occurs in some D&D settings:
 * Adventurer's Guilds are common in Eberron. The city of Sharn has two competing ones, the Clifftop and Deathsgate guild.
 * Shadowrunners and “Mister Johnson” in Shadowrun. In the Genesis and SNES games, your entire party except you was hired temporarily, and random missions were handed out by Johnsons to make money.