Finn's hat

Finn's hat (otherwise referred to as Finn's awesome 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 six episodes: "To Cut a Woman's Hair ," "Heat Signature, "Mortal Folly", "Beautopia", "No One Can Hear You" and "Another Way". It was revealed in "Memories of Boom Boom Mountain" that Finn has had his awesome 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. 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 and also very stretchy since it was able to fit Finn at infancy and adolescence.

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, FYE, Newbury Comics, 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 supposedly numerous hats were distributed at the Adventure Time panel.

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.
 * He is occasionally shown to use the "ears" as actual ears, despite not actually being part of his head.
 * Finn's hat resembles the head of Bueno the Bear from "Barrista," 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 mave 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.