Finn's sweaters

Throughout the course of Adventure Time, Finn is known to possess at least 3 sweaters.

Pink sweater
The pink sweater( like like sweater by finn) is a special sweater knit by Princess Bubblegum for Finn to use during his encounter with The Lich in "Mortal Folly." It is a light-pink sweater with a heart embroidered into it. After initially putting it on, Finn simply remarks on how warm the sweater is, but the sweater proves crucial later as a major weapon in defeating The Lich, due to its being imbued with the power of "liking someone a lot." Finn uses it to tear The Lich's face off, which vanquishes his earthly body. In "Mortal Recoil," Finn tries to use it again when The Lich possesses Princess Bubblegum, but unlike in the previous battle, it fails. The Pink Sweater also protects Finn from the Lich's mind control.

In "Thank You," Finn wears his pink sweater instead of his yellow sweater.

Yellow sweater
The yellow sweater is a sweater Finn often wears into the Ice Kingdom when he must deal with the Ice King. He first wore this sweater in the pilot episode and has since notably worn it in "Prisoners of Love" and "The Chamber of Frozen Blades" (in which Jake reminds Finn to wear it). He hasn't used it since he received the pink sweater.

Holly jolly sweater
Finn possesses another sweater, seen in the 2-part special, "Holly Jolly Secrets." It looks like a stereotypically tacky Christmas sweater. It is dark blue, with yellow dots on the sleeves and near the bottom hem, and it has red moose on it.

Trivia

 * Finn calls his pink sweater his "Like-Like Sweater."