Hackintosh. But it isn't easy OR reliable.
My installation was relatively easy and is incredibly reliable. I installed using a retail legally purchased DVD (Leopard 10.5.0), and updated through Software Update to 10.5.6. I cut HD videos using Final Cut, and I was able to play World of Warcraft with great FPS (before I quit that addictive game).
I currently run mainly Mac OS X, but I also have Windows 7, Windows Vista, and Ubuntu 8.10 as other boot options... I guess you could say my machine quadri-boots

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When I get some free time I'm going to install a second copy of Mac OS X on another partition of one of my drives, as a back-up just in case.
I have virtually no problems. Every now and then my USB devices are not recognized and I have to unplug and plug them back in, and my sleep function doesn't work (the machine won't wake back up). Other than that, you could use my system all day long and never realize it's a Hackintosh.
Check out my video of my system in action:
http://www.youtube.com/watch?v=17YUf5Hef_I
All the videos on that channel were cut on Final Cut Pro using my Hackintosh.
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As for the legality issue, there is some question.
However, it is not "illegal" per se. The only clause pertaining to the issue of installing Mac OS X on a regular PC is along the lines of "This software can only be installed on Apple-labeled hardware."
Well there are all sorts of complications with that statement, if I've got it accurately (which I believe I do). First, what needs to be "Apple-labeled?" The "hardware?" Well, technically we are ONLY installing the software on the HDD itself, nothing more, so does the HDD need to be "Apple-labeled?" What does that mean? If I buy a Mac Pro, I can't purchase a third-party HDD? Is the HDD supplied by Apple in their machines technically an Apple-labeled device?
At any rate, I put an Apple sticker on my tower; it's Apple-labeled.