The Wicker Man is a giant effigy allegedly used by the Druids to perform human sacrifice, according to the writings of Julius Caesar. He mentions it as one method in which the Celts performed rituals, but other than Caesar's observations there is little scholarly corroboration. In today's neopagan practices, a Wicker Man may be used for celebration of a fire feast or at harvest time (although nowadays it's without the human sacrifice).
In 1973, a British horror film called The Wicker Man became popular; it was the story of a policeman who encounters a coven of witches during his investigation of a murder. It was remade in 2006 with Nicolas Cage.
The coven celebrated the end of the harvest with the burning of a Wicker Man.