In Windows 8, the Windows Explorer shell sports a new Ribbon UI.
(Note: Obviously, all of the Ribbon icons shown here are placeholders. These are early days.)

Various Explorer Ribbon tabs.

Examples of dynamic Ribbons in the Windows 8 shell.

A hidden Ribbon (with dynamic Disk Tools tab), and a close-up view of the status-based based view style toggles.

The Explorer Ribbon Find menu.