If you use to-do lists extensively, this free online service is ideal for you. Simply put, Remember The Milk is a simple yet powerful online to-do list manager with several useful features.
Other useful features include:

Locate your tasks. You can use RTM’s map to see where your tasks are located in the real world. By finding out what’s nearby or on your way, you can plan the best way to get things done.
Collaborate. You can share tasks and lists with your colleagues and can also send and publish tasks.
Add tasks easily. Adding tasks is as simple as sending an email. If you have an important date to add, use the Quick Add feature to put it on your list.
Take your tasks with you. You can access your tasks on your web-enabled mobile device. You can print your entire list or a handy weekly planner, which shows upcoming tasks. You can view your tasks on your calendar with Apple iCal or Google Calendar, and subscribe to feeds with Atom/RSS.
Search your tasks. Use the advanced searching feature to find the tasks you want. You also can save your searches as Smart Lists, and see tasks that match your desired criteria.