Dobby's Sock

Dobby's sock was given to him in The Chamber of Secrets by Harry Potter in order to make him a free elf.

Harry Potter tricked Lucius Malfoy into giving dobby the sock, because he knew that Mr. Malfoy would never free Dobby otherwise. The sock was slipped into Tom Riddle's diary, and when Dobby opened it, the sock was technically given to him by his master, Mr. Malfoy.

Appearences 

Harry Potter and the Chamber of Secrets (Movie)

Harry Potter and the Chamber of Secrets (Book)

Harry Potter and the Goblet of Fire (Book)