Introduced In: 10.0 "Cheetah" (2001)
Mac OS X wasn't the first operating system to make aesthetics a paramount responsibility. BeOS, made by an ex-Apple exec, was pretty for its day. And the original Mac OS, when it first launched was considered beautiful. But fast-forward to the top of the aughts, and operating systems were visually drab and unspectacular. Form trailed helplessly behind function.
Then came Cheetah. To complement the new colorful design of Apple's computer's the company decided to build its new operating system with a user interface that was more endearing and easy on the eyes. It settled on "Aqua". Based on water, Aqua featured a number of water droplet-like elements that could be found in the status bars and buttons. Steve Jobs famously went on record saying, "We made the buttons on the screen look so good you'll want to lick them."
Take a look at all the offerings available now, and you'll see that almost every operating system has taken on the translucent, reflective qualities of Apple's much-loved UI.