Google tasks as TODO manager

In Gmail

Shift + t - to create task based on open message

In tasks

Tab and Shift + Tab - subtasks management

Ctrl + Up and Ctrl + Down - move task

Shift + Enter - details

If you set due date of task it will automatically appear in your google calendar

About: https://mail.google.com/mail/help/tasks/

Shortcuts: https://support.google.com/mail/bin/answer.py?hl=en&answer=106237&ctx=mail

Android: https://play.google.com/store/apps/details?id=org.dayup.gtask

iPad: http://itunes.apple.com/ua/app/gotasks-google-tasks-client/id389113399?mt=8

Standalone web app: https://mail.google.com/tasks/ig

Standalone big web app: https://mail.google.com/tasks/canvas

Chrome plugin: https://chrome.google.com/webstore/detail/dmglolhoplikcoamfgjgammjbgchgjdd