Thyself - Mindful mood tracking for greater self-awareness
Future Me - Send an email to your future self
Lemoon - Stay on top of your depression
Letter to Yourself - Send a physical letter to your future self
Nomie - Open-source mood and life tracking
Letter to my future self - Letter to my future self is a prominent email scheduling service that gives you the opportunity to create a letter with goals and thoughts for your future self and schedule a date for sending it.