Taskbar
The strip along the screen edge with one button per open window — Windows 95's most copied idea, which Unix desktops adopted within a year.
Before 1995 a put-away window was an icon somewhere on the desktop, and what you couldn't see you forgot. Windows 95 instead placed a fixed row at the bottom, with the Start button on the left, the clock on the right and in between a button for every window whether visible or minimised. One click switched to it. The parts already existed separately: CDE's Front Panel had launchers and a clock, twm's icon manager had the window list, RISC OS had kept a row of icons along the bottom since 1987. Windows put them together and made the row mandatory.
The Unix world followed immediately. FVWM got a module for it in 1996, kwm and kpanel built KDE around it in 1997, and GNOME's panel arrived in 1999. It is also why EWMH exists: a panel has to be able to ask the window manager which windows there are, and a program has to be able to ask not to appear there, which became _NET_WM_STATE_SKIP_TASKBAR. Windows 7 merged the buttons with pinned icons in 2009, turning the row into a dock, and Windows 11 moved the whole thing to the centre.