Remember The Milk

If your idea of organization centers primarily on to-do lists, this is the application to use.

Remember The Milk is very similar to BackPack, but it has a some different features, focusing more on lists than notes. You can create as many different lists as you want and schedule tasks in real English such as "next month" or "Tuesday 9AM".

If you're a user of the popular David Allen's "Getting Things Done (GTD)" system, this is an excellent companion.

A few things make this online application stand out.

You can add tasks via email.
You can access your tasks with any application that supports the iCal format.
You can receive your lists via Atom feeds.
Receive reminders via email, SMS, AIM, Google Talk, ICQ or MSN
Other features include:

Color-coded priorities
Time estimates
A simple "Postpone" option, to add a day to a task
Repeating tasks
Unlimited notes attached to any task
Archiving
Print your lists
The interface is fast and clean and very intuitive to use. Highly recommended.

http://www.rememberthemilk.com/