A makiwara board, a padded striking post traditionally used in some karate styles, is one of my most recommended types of training equipment for karate practitioners. It is the most recognizable of the "hojo undo", or "supplemental training equipment", used in the Okinawan fighting methods now known as karate. There are a few different type of makiwara, which share the purpose of helping develop correct technique, power generation, and conditioning of the hands, arms, and feet.
http://www.johnstonkarate.com/2013/01/makiwara-board-training-for-karate-pdf.html