VMware fusion is by far my favorite application on my MacBook Pro.
As a kid I grew up running Windows, and I grew to love it, but as I grew older I began to envy the media capabilities Apple offers as I am a musician and wanted the higher capability for music production and such. So when it finally came time for me to get my own laptop I opted for a MacBook Pro, knowing it would mean I had to stop playing all of my PC games, that is until I discovered VMware Fusion.
Although MacBook Pro's come pre-installed with Bootcamp, it is a pain to use as one must reboot their computer to switch between Mac and PC. With VMware Fusion on the other hand it is possible to run both simultaneously and more. It has the ability to run Mac, Windows, and Linux all at once, without losing functionality in any of them. Through VMware you can run the other OS's in separate window, or in unity mode where all of them are merged into one fully functioning OS. I know some people own both a Mac and a PC because they cannot live without certain features of one or the other, but with VMware Fusion that does not have to happen.