Remember The Milk

"Remember The Milk(RTM) is a task management service available in 21 languages and used by over 250,000 people. The main interface to the service is a rich web application, but users can also interact with RTM via email, a mobile version, a public API, iGoogle and Google Calendar gadgets, Twitter, and more." - taken from Google Code FAQ

check it out here www.rememberthemilk.com