If you want simplicity and efficiency then Remindr would be your priority. It has a very clean interface and simple navigation to make it user-friendly as much possible.
An intuitive interface makes managing tasks fun. You can set due dates easily with next Friday or in 2 weeks. It has an extensive keyboard shortcuts make task management quicker than ever. Remember The Milk allows you to receive reminders via email, SMS, and instant messenger (AIM, Gadu-Gadu, Google Talk, ICQ, Jabber, MSN, Skype and Yahoo! are all supported).
Wakerupper is a telephone reminder service that aims to make telephone alerts as simple as possible. Just enter the number of the phone you would like to be called, the time you want the call to be placed, and an optional 140-character reminder message. That’s it – you will receive a call from Wakerupper at the time you have specified and your message will be delivered.
With Google’s free online calendar, it’s easy to keep track of life’s important events all in one place. With Google Calender, you can let your family and friends see your calendar, and view schedules that others have shared with you. You can get event reminders via email or have text messages sent right to your mobile phone.
Task.fm is a reminder tool. Use it to create sms, email or twitter reminders. Task.fm can understand your natural language like “meeting with bob next Tuesday at 9am”, so you don’t need to enter dates and times manually.
HassleMe is a tool which nags you. It nags you via email about things you know you should be doing, but which you’ll forget. You set up a rough frequency, but it actually nags you at random times within certain parameters. It keeps you on your toes.