Google Tasks

http://mail.google.com/mail/help/tasks/
If you have not discovered Google Tasks, you should visit the Gmail
Lab (under Settings) to turn on TASKS.

When you read an email that includes a new item for your to do list, or important information needed to complete that tasks, just click on the Label button to assign this email to a task. Your tasks list will now contain items that are linked to any email related to that tasks.

The most satisfying part is being able to check off the tasks as you complete them. Learn more about how to use the Tasks feature in Gmail or on
your mobile device in Google's Help Section.

Tasks adds a to-do list to Gmail. After adding the lab, click the Tasks link above the chat box on the left-hand-side of the page. Your task list will appear on the right – click anywhere in the list and start typing to add a task.

You can also create a task from an email: from the More Actions menu, choose Add to Tasks when viewing an email, or press shift+T if you have keyboard shortcuts enabled. For more shortcuts, explore the Actions menu at the bottom of your task list.