Rememberthemilk is a service that helps you keep track of tasks and remind you when they need to be done. Tasks can be set as repetative, while adding a location to the task. They can also be sorted according to day.
The service also allows you to add a new task through an email. Tasks can also be tagged to making sorting easier. There is also an advanced search feature which can save searches.
Rememberthemilk reminds you of tasks through email, SMS or even an IM service when its due. Tasks can be shared or sent to other people. The pro account offers the service on iPhone and Windows Mobile Devices. The basic service is free to use and very user friendly.