The Namimori Middle School's gang, run by the Chairman Hibari Kyouya, who appears to own everything in Namimori. They primarily exist to keep the peace in Namimori Middle School, but they also moderate criminal activity across the entire Downtown area. They get their funding from festivals, and destroy any stands that don't cough up the cash, which seems to be an extention of what the Akiyama group used to do. The Akiyama and the Disciplinary Committee tend to coexist and cooperate in all matters.
A yakuza organization based in the Hakuyou region of the greater Namimori area, run by the Kouyou family. Namimori used to have a cult of community based around them, but it's loosened up over the years, especially with the Difo Famiglia's influence. The Akiyama have a number of branches and areas of influence, such as:
The Koyama Middle School, where many kids without futures are funneled into jobs that benefit the group
The Peach Association, a small-time realty and loanshark office based in Downtown Namimori
The Urayama group, a big-time realty office that sells beachside estates in the tourist town of Miyazawa
The Akiyama have a strong association with 'Flames', the magic energy that should be coming from ancient Mafia Famiglia exclusively. Most of the Kouyou family are Suns and Skys. The Akiyama are led exclusively by those with the 'Guiding Eyes'; a genetic-based Sky ability emitted out of the user's eyes that give them immense powers of suggestion. The current heir is Naoki.
The mafia organization Tsuna's father seems to be associated with. Currently being led by the Ninth head. One of the most influential groups in the world, and the one that collects the most Flame-active people. They have multiple branches, including:
The CEDEF, or the "Outside Advisors To The Family", a spy network that both observes and cleans up after missions in areas outside the country, and keeps tabs on other Famiglia. All paper trails are confined within their walls. They get funding from selling information. Sawada Iemitsu is the current boss.
The Varia Squad, an assassination squad that takes requests exclusively from the Vongola, run by Superbi Squalo. Was founded by the swordsman Tyr before Squalo killed him and took over, and then was run by Xanxus for a limited time before he was exiled and control was given back to Squalo. Held together by six main assassins, who each have their own team that help them manage their missions in some way (including non-hit commissions).
The current heir is Federico Fiore, a kindergarten teacher who is forced into taking on the group thanks to the death (and exile) of his brothers, and he's being managed by Reborn, a friend of the Ninth's.
A mafia orginization based in Sicily, but controls every single route into Asia, and has been since the 1800s. Has major branches, but all under one name and authority. Mafia groups cannot operate in Japan without Difo approval. The Difo are great friends with the much stronger Vongola, who own multiple properties in Japan, and often take their side in most situations.
A smaller mafia famiglia known for their collector's relics, currently being run by the Fourth, Romolo Zeni, who is also an archaeologist. Despite the surname and the absolute influence, Zeni is actually a cousin to the main bloodline, who haven't been able to reclaim the family, considering Zeni is basically immortal. The family is nothing but infighting. Not even Vindice is going to touch that.
"The Vengeance", also known as "The Sinner's Curse", a group of bounty hunters who capture people, try them in their own court, and jail them as appropriate. They have their own maximum security prison for troublesome personalities. Covered in bandages and powered by Hatred and Despair. The leader, Bermuda, appears to be looking for something related to Gi U and "The Green Door Project".
The Green Door Project
A mysterious Flame-based research endeavour that involved human experimentation that met an unfortunate end. Gi U is one of their subjects. All records of their work have been purged, but they sunk their teeth into a lot of places...
Currently, the only known experiments are the other children who were being tested with Gi U. Gi U hates every single one of them and refuses to find them, even when repeatedly prompted.
A tiny mafia famiglia that seems obsessed with researching Flames and their relationship to spacetime, creating reality-warping weapons. Probably a cult. Lambo was part of their family. Their scientific breakthroughs count by the dozens, and they apparently have some form of energy source powering their research.