HabitForge (personal improvement)
Based on the idea that a chore performed 21 days in a row becomes a habit, HabitForge lets you set a habit that you want to develop and sends you an email the next day to ask whether you did the task. Complete a task for 21 days in a row, and you get a congratulations email. You can develop habits alone or with groups. Users of the free, ad-supported service can set only one active public habit at a time.
HabitForge | Free; $10/year for the premium version