The term used for obsessive fans of a particular activity that goes a bit too far onto the creepy side. Unlike fangirls (which usually refers to women obsessed with characters and their relationships with either other characters or themselves to the exclusion of the rest of the elements of the canon), the term "fanboy" is usually reserved for antisocial people with a self-inflated ego over their knowledge and/or mastery of the topic in question, often parading it around and belittling those around him. They are to fandoms what "Stop Having Fun" Guys are to tournament rules.
In Japanese, the term Otaku is synonymous with this, but also implies an almost stalker-like attitude as well. The original (very pejorative) meaning got lost when it was adopted by English language fans of Anime.
See also Ascended Fanboy, Promoted Fanboy, Fan Dumb, Saw "Star Wars" Twenty-Seven Times, and Running the Asylum.
Mikami of Death Note is a Kira fanboy that would do just about anything for "God".
Mitsuru from Battle Royale is a Kiriyama fanboy, that promised to serve him after being saved from a beat down.
Graham Specter from Baccano! is the No. 1 Ladd Russo fanboy.
Tamaki from Code Geass is a member of the Black Knights, who's always Fanboying about his "best buddy" Zero. Despite not being very intelligent, he has the most trouble believing Schniezel's lies about Lelouch.
And Diethard. So much that, when dying of a gunshot wound, he asks Lelouch to kill him with Geass.
One Piece: Bartolomeo is a sociopathic asshole who may very well be even worse than Eustass Kid — and a massive fanboy of Luffy ever since Loguetown, an event that would eventually lead him to becoming a pirate. Ironically, the same moment that would make him into a fanboy of Luffy is also the same moment that caused Smoker to chase after Luffy in order to catch him.
Akito Tenkawa and Gai Daigouji of Martian Successor Nadesico are such huge fans of Geikiganger 3 that they even use attacks and battle cries from that show when fighting the Jovians.
Patton Oswalt's character Paul in Big Fan. Unlike most fanboys, Paul does not obsesses over Sci Fi, Fantasy, Comics, Anime, or Video Games, he is a football fan. He was beaten up by his sport hero, and does not press charges in order to support his favorite team The Giants.
Free Enterprise is a whole film devoted to this. It follows two fanboys who fulfill the nerd dream of striking up a friendship with William Shatner, only to find out he's as much of a loser (in-universe, mind you) as they are.
During the "iStart A Fan War" episode of iCarly, there is one fanboy for the Sam/Freddie 'Seddie' Shipping couple who is a textbook version of this. He stands up and shouts "SEDDIE!" over and over. For those with knowledge of the fandom, he's obviously based on one specific Big Name Fan who became well known for those antics on the creators blog and websites.
Fanbot from VR Troopers. Although mostly comic relief with his Hollywood fanboy personality and proved to be a bit clumsy, he was actually the most powerful monster of the week they faced all season. Before fighting the Troopers, he was assigned to destroy a traitorous ally's super powerful robot, which he did fairly easily. JB's normal finishing move (the laser lance) was so ineffective that be it impalement, slashing, you name it, it only tickled to the point where he made a joke about it. JB actually had to hit him with the techno-bazooka to defeat him, a weapon used against multiple aircraft (but repowered his laser lance and attacked the already defeated robot for some reason). His personality was lampshaded when JB said during the battle "sorry, no autographs" and "there's nothing worse than an angry fan". He actually wasn't a fanboy by designed but a giant industrial fan (he killed the other robot by sucking his parts through his blades), so his name was a pun on fan.
The character of "Whizzkid" in the Doctor Who story "The Greatest Show in the Galaxy" is widely-seen as a demonstration of how the show's creators at the time regarded fanboys. Especially his rapid and horrible death.
Solek from BIONICLE is a Kopaka-fanboy, to the point that he changed his color to white to look more like his idol. When they meet, Solek's overjoyed and starts asking about his legendary adventures, which Kopaka doesn't remember due to amnesia.
The Adoring Fan of The Elder Scrolls IV: Oblivion was a character who would appear after you became the Arena Grand Champion and follow you everywhere. His obsessive devotion to your character has led many people to try to kill him by any means necessary, and his tendency to respawn only made him more annoying.
Dr. Hal Emmerich, in the Metal Gear universe, is a brilliant scientist...who self-identifies as an Otaku. His code name, Otacon, is derived from Otakon, the Otaku Convention. (This was done with the con's permission; in return, their program each year lists "Dr. Hal Emmerich" as their "Scientific Adviser.")
The fictional TV showSteel Samurai in the Phoenix Wright: Ace Attorney games has a few of these, as well as its spin-offs like the Nickle Samurai and the Pink Princess and rival show Jammin' Ninja. Maya is a huge fan of all of these and is always dragging Phoenix into the mix, and in the third case of the first game, we meet Cody Hackins, a kid who obsesses over the show so much that he photographs all of the Steel Samurai's finishing moves and refuses to believe that the Steel Samurai would lose a fight/get murdered. Cody's Japanese name, Kyuuta Ootaki, is a similar to that of Otaku.
The biggest Steel Samurai fanboy in the series, however, is Miles Edgeworth. It's subtle enough in the other games, but it's Turned Up to Eleven in Edgeworth's own spin-off game. At one point he even tells someone they're not fit to call themselves a true fan of the Steel Samurai because they claim to have gone to the bathroom during the climactic scene of the play.
And oh, does the hilarity ensue when Edgeworth finds out the guy under the Steel Samurai costume who he's been fanboying all evening is Larry Butz.
Darth on 6teen is a Star Wars fanboy. He wears a cape and a Darth Vader helmet, carries a red lightsaber, and is part of the Jedi Knights club. Oddly, one of the "clones" (mean girls) is also in the Jedi Knights club, An Aesop about how you shouldn't label people.
On Almost Naked Animals, Howie is so obsessed with a stuntman named Dirk Danger that he hangs out in Dirk Danger's cave and tries to coax him out of retirement.
Fanboy and Chum Chum were clearly meant to be this, but aside from wearing superhero costumes and occasionally being seen reading comic books, they don't exhibit typical fanboy behaviour. Oz, their acquaintance and the owner of the local comic book shop, is a textbook example.
Of the three Star franchises, Trek, Wars, and Gate, Richard Dean Anderson's is his third favorite.
Control Freak of Teen Titans. Aqualad even calls him out on it.
One episode of The Cleveland Show has Cleveland get into an Edit War with someone over the grams of fat content of a microwave snack. The war escalates when the other fan in question slanders Cleveland's image. When Cleveland finally meets the person slandering him, the person is revealed to be an obese pre teen who admits to eating the snack 5 times a day because he loves the snack and is a true fan.