Light's watch

The watch Light Yagami receives as a present from his father after entering university serves an important plot-device in the series.

When Light loses his memories by relinquishing his notebook, he plans to regain the ownership by killing its new owner. In order to do this, he cuts a piece of the Death Note and places it in a compartment that opens from the bottom of his watch. The compartment only reveals itself when he pulls the crown of his watch four times with less than a second in-between pulls.

The watch helps him kill both Higuchi and Takada. In the final stand against Near, Light attempts to use a piece of the Death Note to kill the former, but his plan is thwarted by Matsuda.

Trivia

 * In the films, Light only needs to pry the watch open in order to reveal the compartment.
 * In the L: Change the World novelization, it is revealed that following Light's death, L took his watch, which broke during their last confrontation, and kept it as a memento.
 * Although dying in different times relative to their continuity, it is interesting to note that in the manga, anime and the films, Light kills Takada using a piece of the Death Note hidden in the watch.