Jobs generally are posted on a bulletin board (which may be an encrypted dataserver or a cork slab, depending on the setting), where certified adventurers sign up for them. "Certified Adventurers" is basically a euphemism for "freelance mercenaries" or, in more cynical terms, "murder hobos". The Protagonist will commonly be one because it is a convenient frame narrative for introducing side quests and plotlines. It can be used for everything from the Knight Errant to the Psycho for Hire to hidden royalty.
If Craigslist had a section labelled "Jobs wanted: Medieval Commando Squads", it would be one of these.
It is 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.
Real-life guilds were established to safeguard their members from competition and outside economic forces and had official license from the government to be the sole tradesmen in a city; despite this, fictional guilds will often form rivalries and compete with each other in a quasi-market economy. This furthers perception of them being more companies of mercenaries than an actual "guild".
One more thing: Although the main characters are commonly 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. Also compare Hero Academy, which focuses primarily on teaching students in successfully dealing with quests like those mentioned above (especially if such quests are strictly heroic) though may have some elements of Adventure Guild.
- 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.
- 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: Various Guilds and Factions are joinable in each game starting with Daggerfall. They tend to be more function-specific and function more like actual medieval guilds than the odd job clearinghouses that epitomize this trope. The Fighters Guild comes closest to the standard definition of the trope, offering tasks ranging from low-end critter killing to being Bounty Hunters to protection services to, at the high-end, dealing with supernatural threats. The Companions take this role instead in Skyrim.
- 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
- Agarest Senki 2
- Quest Cards in the first Fable game.
- The eponymous Star Wolves are a freelance starfighter squadron who make their living this way.
- 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. In Phantasy Star Universe they were renamed Guardians but essentially do the same killing of monsters and handling of odd jobs. In Phantasy Star Online 2, they are rebranded as ARKS, with their duties expanded to combating Darkers and investigating planets to determine their viability for colonization.
- In Solatorobo, the adventurers are called Hunters, but there are many guilds, the largest of which are the Kuvasz.
- The Bracer's Guild from The Legend of Heroes: Trails in the Sky. The lore goes on to explain a bit of how they function: they're a non-government organization that's allowed to operate among the nations who agree to host them. They do follow regulations, though: while they have legal authority to perform arrests, they are not allowed to arrest politicians, and they are not allowed to break any laws of the nation they're in while carrying out contracts.
- Vector Thrust boasts organisations of dubious legality like Bastille and Heron, which will get mercenaries an airbase to call home and a steady stream of contracts in conflicts around the world. Most of them specialise in the lawless territories of the Kingdom
- Xenoblade Chronicles X has BLADE, the military force of New Los Angeles. BLADE, itself, is divided into eight "Divisions" which specialize in different tasks and provide their own unique benefits to their members:
- Pathfinders specialize in exploration and installing data probes. They have improved HP recovery.
- Interceptors specialize in combating indigens and completing missions. They receive a buff to ranged damage.
- Harriers are combat specialists tasked with fighting indigens and tyrants. They receive a buff to melee damage.
- Reclaimers specialize in treasure box acquisition and data probe installation. They have improved drop rates.
- Curators specialize in combating tyrants and collecting items. They have improved critical hit rates.
- Prospectors specialize in collecting resources and exploration. They receive a buff to their defenses.
- Outfitters specialize in aiding in Arms Manufacturer R&D and collecting Miranium. Their earnings from mining probes are increased.
- Mediators specialize in quest completion and aiding other BLADEs. They receive TP from using Arts in addition to normal attacks.
- The protagonists of Rakenzarn Tales all eventually belong to the Nutsy Guild of Densel Town. Besides serving as a base of sorts, it's also where you can access the game's sidequests.
Non-Video Game Examples
- Wizard guilds in Fairy Tail are this, rather than a Magical Society you might expect. This is because magic in this setting is informal and unique to the individual, and its practitioners are basically superpowered heroes for hire rather than Robe and Wizard Hat types.
- 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.
- High Level Familia such as Loki Familia operate as this in Is It Wrong to Try to Pick Up Girls in a Dungeon?. In addition to slaying monsters in the titular dungeon they very frequently take on quests to retrieve drop items or rare materials from the dungeon, especially the lower levels, or act as escorts for members of the Familia more focused on crafting to gather the materials themselves. The Guild (which is not this, despite the name) can also issue quests directly, such as commanding Familia to participate in the defence of the city against would be invaders.
- These exist in the various cities of E-Rantel in Overlord, both taking requests for various missions to fight against the monsters of the world as well as setting prices and overseeing the ranking system that governs individual adventurers.
- The Pathfinder Society is this trope for Pathfinder. The society is dedicated to explore Golarion, investigating ruins, recovering tomes and artifacts as well charting the world. Not confuse with Pathfinder Society, the official organized play campaign where the PCs are members of the eponymous organization.
- 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.
- Black Star in Shadowrun was a militant wing of the Anarchist Black Cross that provided training and support for the titular Shadowrunners, originally Neo-Anarchist mercenaries who were paid by the Mega Corps to sabotage their competitors and inadvertantly chip away at the system they had created. However, Black Star was decimated in 4th edition during the Az-Am War, and "modern" Shadowrunners tend to be freelancers only in it for the money.