You may need (or want) to run Windows, or other operating systems, alongside Mac OS X, and Parallels Desktop (4 mice) is the best-known of several programs on the market for that purpose. (Full native Windows support, of course, is also available via Apple's Boot Camp, but it requires you to reboot out of OS X and into Windows.) A new-to-the-Mac player now brings a formidable challenger to the arena, however. VMware, an expert in x86 virtualization—that is, the ability to run one or more x86 operating systems as 'guest' under a 'host' x86 operating system—has released Fusion 1.0, its first OS X offering. Like Parallels, Fusion allows you to run many versions of Windows and other operating systems from within OS X. And unlike Boot Camp, you don't have to log out and restart in order to use it.
VMware Fusion supports more than 60 operating systems: Windows coverage extends from version 3.1 to betas of Windows Server 2008. If Linux is your cup of tea, you'll find support for Red Hat, Ubuntu, SUSE, Mandrake, and more. You can also install Novell Netware, Solaris 9 or 10, FreeBSD, and MS-DOS systems. Even 64-bit releases of Windows and some families of Linux, such as Red Hat and SUSE Enterprise Linux, are supported.
Read my Macworld article, Review: VMware Fusion, for the rest of the story...