To start us out (From Wikipedia):

One interesting historical aspect of the classic Mac OS was a relatively unknown secret prototype Apple started work on in 1992, code-named Project Star Trek. The goal of this project was to create a version of Mac OS that would run on Intel-compatible x86 personal computers. It was short lived, being cancelled only one year later in 1993 due to political infighting, though its team was able to get the Macintosh Finder and some basic applications, like QuickTime, running smoothly on a PC.

Although the Star Trek software was never released, third-party Macintosh emulators, such as vMac, Basilisk II, and Executor, eventually made it possible to run the classic Mac OS on x86 PCs. These emulators were restricted to emulating the 68000 line of processors, and as such couldn’t run versions of the Mac OS newer than 8.1, which required PowerPC processors. Recently, the PearPC emulator has appeared, which is capable of emulating the PowerPC processors required by newer versions of the Mac OS (like Mac OS X). Unfortunately, it is still in the early stages and, like many emulators, tends to run much slower than a native OS would.

Apple later began a similar project for Mac OS X, code-named Marklar. Marklar’s task was to keep Mac OS X and all supporting applications (including iLife and Xcode) running on the x86 architecture as well as that of the PowerPC. Marklar was revealed by Apple’s CEO Steve Jobs in June 2005, when he announced that future Macintosh products will run on Intel processors starting in 2006.