37signals has an interesting article about how they use Parallels Desktop:
Parallels is an indispensable tool that lets us test our applications in all the browsers we support on a single computer. We can even test IE 6 and 7 side-by-side by running Windows XP in one VM and Vista in another. As a web developer, Parallels is easily the single best reason to own an Intel Mac.
The article is mainly a criticism of Microsoft’s Windows Vista, but still, it’s an interesting, albeit brief, look at how VM software such as Parallels Desktop is being used by web designers.