bbc.com/.co.uk is a major website with a collapsible header nav section - triggered by a section of the main nav labelled "More v". The main nav also overflows into that section when used on smaller screens, but several of the links are always in there. The main nav / collapsible section is common across the entire site, pushing the content of the individual pages down when it is opened.
Outside of websites, there could also be an argument that the ribbon toolbar is a collapsible sub-nav section. The justification for the ability to collapse there was also to increase the amount of screen space available to users for the main document workspace.
One of the neat things about the Ribbon is that it can be closed. For times when you want the maximum concentration on your document, you can collapse the ribbon to just a single line. Buttons you have added to the Quick Access Toolbar (the area at the top where the Save button is) stay around, as well as access to the Floatie (watch the video or stay tuned!) and context menus. So, sometimes when I'm just trying to get ideas down on the page, I collapse the ribbon and use my whole screen for typing. Then, when I'm beautifying and laying out my document for others to see, I expand the Ribbon and use the full functionality of Word to get the output I expect.
Jensen Harris - Enter the Ribbon