Google has rolled out Task list to Gmail through labs. With this feature enabled, users can add emails to task list. You get better control of your emails with tasks feature. To access this feature, you need to enabled it through the labs. You can also add a mail to the task list by Shift+T if you have keyboard shortcuts enabled on your account.
To add an email to the task list, select the email and from the more actions menu, select Add to Tasks. You can also use the keyboard shortcut Shift+T if its enabled on your account.
Once the to do list is open, you can mark off completed items, rearrange your list, or perform all sorts of other functions.
Source: Gmail Blog