Google Calendar, Drive, and Mail icons are located on the top right corner of the page. Click the icons to easily navigate between all three. For all you productivity wizards out there, you can use our handy-dandy keyboard shortcuts to slide between accounts with a few simple keystroke.
If you'd like your Drive files to open within Shift, head into Settings, Accounts, and toggle on "Open Google Docs in Shift". This will open all Drive files in a new Shift window, instead of in your browser.