Why you need this
Today most screens are wide or ultra-wide. Vertical screen space is precious. Every extra toolbar or navigation menu reduces the available screen space. Google design is particularly bad: in addition to OS-level and browser-level horisontal user-interface elements (task bar, window title, tabs, address bar), Google adds at least three more bars in most of its products (+You menu, search bar, and context-sensitive tool bar). The first two are rarely used, but occupy as much as 9% of a typical modern screen. Install this script, and enjoy Gmail as if you had a bigger screen.
Minimize Navigation Bars in Gmail script hides +You menu and search bar in Gmail interface. It places a new narrow button which allows to show hidden bars when you need them (like if you want to search in Gmail). When you don't need them, the same button helps to collapse Google tool bars again.
<img src="http://i.imgur.com/uxjts.gif" alt="screenshot" style="max-width: 100%;">