Finn's hat

Finn's hat is a piece of headgear that Finn wears throughout the Adventure Time series. It has come to be his trademark of sorts, as it is arguably his most distinct attribute. The hat is white and covers his entire head except for his face, and also has little bumps on the top at both sides that resemble bear ears. In the original short, the hole for his face was a semi circle, and in the series it is an oval. Finn wears his hat almost constantly, and has even been shown to wear it to bed. When he removed the hat in "To Cut a Woman's Hair," Jake seemed surprised at his hair, so it's likely that he rarely removes the hat not even to cut his hair, and if he does, it is in seclusion. Finn has removed his hat, so far, in nineteen episodes: "To Cut a Woman's Hair," "Heat Signature," "Mortal Folly," "Apple Thief," "Beautopia," "No One Can Hear You," "Another Way," "Beyond this Earthly Realm," "Gotcha! (in a reflection)," "You Made Me," "Who Would Win," "The Hard Easy,"  "The Lich," (farmworld version), "Jake the Dog," "All the Little People," "Davey," "Little Dude", "The Great Bird Man and, "A Glitch Is a Glitch"." It was revealed in "Memories of Boom Boom Mountain" that Finn has had his hat ever since he was an infant. Pendleton Ward has stated that Finn got his hat as a baby when he skinned a bear out in the woods.This is possibly confirmed in the episode "Little Dude", where Finn remarks at the end that he needs a new hat, and Jake replies that they need to go skin an evil bear.

Finn appears to be quite defensive when it comes to his hat, for when the Ice King states that Finn's hat is stupid in the "Animated Short," Finn retorted by kicking Ice King in the face and yelling, "My hat is awesome!" It is presumably unique. In "Little Dude" the hat became enchanted when it touched the flower of a buried Magi. The evil spirit that possessed it caused the hat to rampage through Candy Kingdom. After its deeds were stopped, the hat was enchanted with a good spirit and given to the Magi by Finn, who later, with Jake, went off to get another hat from an evil bear.

Real life Finn hats
The Frederator Blog revealed that Cartoon Network's Creative Services division distributed a number of Finn hats to promote the show. The only stores known to carry the hats are Hot Topic, Toys R Us, FYE, Newbury Comics, Thinkgeek, Comic Alley and Spencer's, but other hats might be given away in future contests. The blog also has photos of the show's crew and numerous fans wearing their very own hats. At Comic-Con, numerous hats were distributed at the Adventure Time panel. Also, a Finn hat is included in the Jake Vs. Me-mow DVD set. The hats for sale have straps at the chin, unlike Finn's actual hat which can just slip on and off. This is likely to adjust for variying head sizes.

Trivia

 * In the "Animated Short," the design for the hat was different, as the hole for his face was a half circle rather than the ellipse seen in the show.
 * It is the only object that has appeared in every single Adventure Time episode, including the "Animated Short﻿." The only episode in which it has not appered has been the Fionna and Cake episode "Bad Little Boy."
 * Finn is occasionally shown to use the "ears" as actual ears, despite not actually being part of his head.
 * Finn's bear-ear hat resembles the head of Bueno the Bear from "Barista," which is a short by Pendleton Ward. (For this reason, it is sometimes referred to as a "bear hat.") The hat also resembles Max's hat from Where the Wild Things Are and Maxwell's rooster hat from Scribblenauts.
 * The two "ears" on Finn's hat are portrayed as bear ears in FusionFall.
 * According to Pendleton Ward, the hat is attached to his chin instead of his neck.
 * Its origins will be explored in later episode pitched by Andy Ristaino.
 * In the episode "In Your Footsteps," the bear is seen wearing a hat like Finn's. This implies that he may have multiple hats, and changes them out when they are marred. This also may mean that he no longer wears the hat he had as an infant, or if he can even still fit it on his head.
 * In the episode King Worm, Finn's hat has longer ears in some scenes to signify when he is dreaming.
 * Finn's hat was destroyed in "You Made Me"
 * The Hyooman tribe wears hats similar to Finn's, but modeled after different animals and are much more detailed.
 * Finn's hat resembles a polar bear.
 * Fionna's bunny-ear hat that she wears in the episode "Fionna and Cake" resembles Finn's hat in a different style.
 * Make your own Finn hat from the Frederator blog, including a video tutorial and fabric patterns(Has been taken down).
 * The evil little dude is a reference to the "headcrab" creature from the videogame "Half-Life," made by Valve.