Light's watch

Light's watch is a watch that Light Yagami receives as a present from his father after entering university, and it serves an important plot-device in the series.

Looking closely on the watch in the books, the watch that is shown is a OMEGA(r) Speedmaster Professional.

Plot
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 novel L: Change the World, it is revealed that, following Light's death, L took his watch (which broke during their final 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.