Curiosity is a well-studied psychological phenomenon, an important human virtue, a motivating factor underlying learning, an enabler for self-improvement and an impetus for empathy. Curiosity is also a powerful motivating factor designers can leverage to increase user engagement, particularly in applications requiring long-term interaction (e.g., tutoring systems, health tracking tools). In these projects, we explore ways to design systems and agents to elicit and sustain curiosity in end users.