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.

Plot
When Light loses his memories by relinquishing his notebook, he plans to regain ownership by killing its new owner. In order to do this, he cuts a piece of paper from 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 is instrumental in his plan to wipe his memories and regain the Death Note. Because the watch is a gift from his father, Light keeps the watch on him at all times and knows he will do so even after his memories are erased. When Higuchi, the acting Kira at the time, is caught, Light discreetly kills Higuchi while holding onto the notebook in order to keep his memories and regain ownership.

The watch's mechanism is used by Light to subtly kill Takada and make her set fire to her surroundings in order to get rid of any evidence that she killed Mello with paper from the Death Note. Light's subordinate, Mikami, not knowing about Light possessing a snippet of the Death Note serves as their undoing when Mikami is forced to take out the real Death Note, exposing it to Near.

In the final stand against Near, Light attempts to use a piece of the Death Note to kill the former as a last resort, but his plan is thwarted by Matsuda after he shoots him. The snippet is then confiscated by Aizawa.

L: Change the WorLd novel
In the novel L: Change the World, L wears Light's broken watch throughout the story as a memento.

In one scene, L and Maki Nikaido talk about Kira and how L failed to catch him since Kira died. L wraps his hand "protectively" around the watch. Maki asks him what he's thinking about, and L says, "I was thinking about my only friend." She asks if the broken watch belonged to the friend, and L says yes.

Trivia

 * In the films, Light needs to pry the watch open in order to reveal the compartment.
 * 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.
 * Yuki Shien uses a similar watch in the film Death Note: Light Up the NEW World, presumably in homage to Light.
 * The watch appears to be an OMEGA (r) Speed-Master Professional; however, in the anime, it resembles it slightly less. The same OMEGA watch is famous for being the first watch to be worn on the moon, which has some relevance to Light's name in Kanji.