Sarah's Spy Will is her version of something that every sensible government agent makes out, for use in the event of their death in the field. A spy will is usually contained in a small circular box, opened by a stick-like key. The spy will goes to the spy's family and friends or loved other ones in the event of their death.
We see in an episode that Sarah gives her spy will contained within its silver lock box to Chuck, since she says that he is her new life. It is believed that Chuck is still in possession of this will. Sarah giving Chuck her will prompted him to make one of his own.