Updated to clarify that 10.0 was the first official client version of Mac OS X, and that Mac OS X Server shipped prior to its release.
On this day, December 20, just fifteen years ago, Apple paid $400 million to purchase NeXT. That's the move that brought Steve Jobs back to the company he helped to found, and it was the beginning of the incredible run of success that continues to this day.
According to CNET's coverage on that day, the company was shopping for an operating system to replace the aging Mac OS. It had recently abandoned the Copland system project and was negotiating with Be and other companies. With the purchase of NeXT, Apple bought the core of what is now Mac OS X. It also gained WebObjects, the powerful Web app development tool that had been licensed to 275 corporate customers by the time of the sale to Apple.
In addition to WebObjects, Apple also gained the powerful programming environment known as OpenStep. That was the core of the NeXTSTEP operating system, and is the heart of what became Cocoa for Mac OS applications. In fact, Cocoa classes start with the acronym NS, which stands for either the NeXT-Sun creation of OpenStep or for NeXTSTEP.
In retrospect, it's humorous to look at then-chairman and CEO Gil Amelio's comment that "Apple expects to ship products with the NeXT operating system in 1997." The client version of Mac OS X didn't actually ship in a non-beta version until 10.0 "Cheetah" hit the market on March 24, 2001. Mac OS X Server 1.0 shipped in March of 1999, but it wasn't really recognizable as the Mac OS X we know today; it was a hybrid of NeXTSTEP UI elements and Mac concepts, and lacked the Carbon APIs.
That $400 million dollar investment paid off in spades for Apple, both in the technology that it acquired and in bringing back Steve Jobs to save the company. It's hard to believe that everything that's happened in Apple's modern era started just 15 years ago, but it's true: OS X, and eventually the iPod, the iPhone, and the iPad were all made possible by Apple's purchase of NeXT, and Steve Jobs' return to his company.