Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
Memo to Me - Online email & SMS reminder service.
TickTick - TickTickis a cross-platform to-do list app & task manager helps you to get all things done and make life well organized.
Simply Remind Me - Free email and text reminder service so you don't forget the important things in life!
Remember The Milk - Remember The Milk is a task and time management application for mobile devices.
Kana Reminder - Kana Reminder is a program which can be used to set a reminder to be triggered at a specified time.