Though very happy with Parallels, I have been interested in the performance of the main competition, VMware Fusion.
Get the best of both the Mac and PC worlds with VMware Fusion. With an intuitive Mac-native interface and a wide array of powerful features, VMware Fusion provides the most seamless way to run Windows applications on your Mac.
I don’t do a whole lot in Windows, but for some stuff you need Windows such as GO!View for the PSP.
These OEM versions will work with VMware Fusion (and Boot Camp or Parallels). Though as OEM versions you won’t get any support from Microsoft, for that you will need to purchase the full retail versions.
So here I am installing Leopard on an external drive on my iMac and installing various applications. One application I use now and again is Parallels so that I can run Windows and Ubuntu on my Mac in a virtual machine.
One of the things I like about running an OS in a virtual machine is that I can just copy them from the internal drive to the external drive and do not need to worry about installing Windows again, basically it just remembers where it was when I closed it down previously – even though that was on a different drive running a different version of OS X.