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 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 pull.

The watch is instrumental in his plan to wipe his memories and regain the Death Note, as he needs it on so that he could kill the current owner while holding onto it, so he kills Higuchi using the watch.

The watch is used with Takada in order to remove the evidence on her. 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, who shoots him multiple times in an outrage, before the snippet is 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.