Start menu
The button in the corner of the taskbar that Windows 95 made the gateway to everything, so obvious that Microsoft once lost it and had to bring it back.
Windows 3.x had Program Manager, a screen full of group windows full of icons, and the usability tests during the Chicago project showed that novices could not find their programs in it. The answer in Windows 95 was a single button labelled Start at the bottom left of the taskbar that popped up a menu: Programs as cascading submenus, Documents, Settings, Find, Help, Run and Shut Down. The advertising ran to the Rolling Stones' "Start Me Up", and the fact that you click Start to turn the machine off was a running joke by that autumn.
Windows XP gave the menu two columns with recently used programs, Vista a search box that in practice replaced browsing. Windows 8 removed the button in 2012 in favour of a screen of tiles, and the protests were loud enough that 8.1 restored the button and Windows 10 the whole menu. A menu in the bottom-left corner was copied by KDE's K menu, GNOME's panel and most Linux desktops; the CDE front panel had much the same thing two years earlier, without anyone outside the Unix world noticing.