Google Mail - Collapsed left sidebar
Working yesterday, my left sidebar was collapsed when in email. Not having seen this before, I had to search to see how to display it again. Though, if you are working on a smaller screen, this is a nice feature to know about when screen real estate is needed for other tasks! Here is an example of the sidebar displayed (on the right) vs collapsed (on the left): You can see how it pops out when you mouse-over the icons: Use the "hamburger" icon at the top to turn this feature on/off: