Remember the Milk is a To Do list and manager which is now integrated with Twitter. Here is how to use it (from their web page)

"Getting started

If you're not already using Twitter, you'll need to sign up for an account. You'll also need to add rtm as your friend on Twitter (either by clicking the 'add' link on rtm's profile page, or sending the message 'follow rtm' to Twitter). Then, complete the quick and easy set up process in the box on the above right.
Interacting via direct messages

You can interact with Remember The Milk via direct messages on Twitter. To add a task to your account, just send your rtm friend a direct message with the name of the task. You can also send one of the commands listed below to do more with your tasks.

If you're sending a message to Twitter via SMS or IM or the web update box, remember to add d rtm before your task name or command to indicate that it's a direct message. For example:

d rtm pick up the milk

d rtm !complete call jimmy

(If you're sending the message through the direct message web page on Twitter, you can leave off the d rtm part.)
Command list

You can direct message the following commands to rtm. You may receive a confirmation or info back via a return direct message.
Adding tasks

pick up the milk adds a new task with the specified name to the Inbox

call jimmy at 5pm tomorrow adds a new task with the specified name and due date to the Inbox

return library books in 2 weeks adds a new task with the specified name and due date to the Inbox
Sending tasks

@username pick up the milk sends task to the specified Twitter username (this person must be set up with Remember The Milk too)
Updating tasks

!complete call jimmy completes the specified task (shortcut: !c)

!postpone call jimmy postpones the specified task (shortcut: !p)
Getting tasks

!today gets tasks due today (shortcut: !tod)

!tomorrow gets tasks due today (shortcut: !tom)

!getdue friday gets tasks due on the specified date (shortcut: !gd)

!getlist personal gets tasks from the specified list (shortcut: !gl)

!gettag call gets tasks with the specified tag (shortcut: !gt)

!getlocation office gets tasks at the specified location (shortcut: !go)
Setting up

!on enables task reminders

!off disables task reminders

!help gets help info

!tips gets a list of commands
Getting reminded

Once you're set up with Twitter, you can receive task reminders as direct messages to your Twitter account. These will be sent according to your general reminder preferences (more details on reminder settings are available in the Getting Started guide). To disable reminders on Twitter, send a direct message with !off to rtm, or toggle this option on the Remember The Milk 'Settings' screen under the 'Reminders' tab."