I love virtualization, mainly because I am to poor to buy lots of computers. Here are some of the better options for those of us on a limited budget.

If you need or want to run OS X but only have an x86 machine give PearPC a try. I have run OS X on my Intel laptop and it works very well and is fast. PearOC is GPL software.

I currently use VMWare to run Windows XP under Linux, because there's that one damned program I can't live without! Windows XP runs almost native! I render Poser and Cinema 4D models in Windows running in VMWare and it is no different than if Windows were installed directly on my 3ghz HT with 1.2gb ram laptop.

If you're in need of Enterprise level server virtualization check out Virtual Iron. I haven't used it personally, but it looks really good! They say they are the alternative to VMWare.

There are others, like ZenSource which is free for non-commercial use. It maybe good, but VMWare does what I need so I doubt I will play with it. You may also want to look at Qemu , which is a GPL solution.

I have tried a few on Mac PPC, but typically they are crap and cost you. There is Parallels which will cost you, but this I have not tried. Maybe it's good I don't know, but Vitual PC absolutely sucks. It's just two slow, even on my wife's Dual PPC G5. It may run better on newer Intels, don't know that either. VMWare is releasing something for Intel based Macs so I would hold off until then. My experience with everything else has been bad and VirtualPC is the best that I am aware of to date. We did try Wintel, which was so slow I thought I went back in time to 1992...

I never really understood why an OS X user would want to run Windows since they have many of the same programs, but I guess if you're switching there maybe something. If you have an Intel based Mac and need to run a Windows app or two Crossover may have the answer and you will not have to install Windows at all. Check out Crossover Mac and If you're a gamer you will want to look at Cider.

If you're a Linux user Win4Lin may look good and may work good for you, but I thought it was slow and a pain to setup.  Personally, I really like VMWare the best.