PastPost - Send a letter to your future self
Journaley - A simple and elegant open-source journal keeping software for Windows compatible with Day One
Future Me - Send an email to your future self
Daybook - Daybook is an application that allows you to record activities, experiences, thoughts, and entire ideas throughout a day and protect all of your information with a strong password.
Letter to Yourself - Send a physical letter to your future self
GloboNote - GloboNote is a free and easy to use desktop note taking application.