VMWare recently released the second major revision to its virtual machine application, VMWare 2.0. VMWare, which runs natively in Windows NT and 2000, provides a PC emulation environment in which you install and run other operating systems, such as Windows 98, Linux, or MS-DOS. The appeal of VMWare is largely for developers that need to target a variety of environments, though the emulated systems, which can run in a window or full screen in the host OS, run at only a fraction of the speed they would natively. However, most people can't afford multiple systems or the downtime of dual booting, so it does satisfy a need.
VMWare 2.0 includes many new features, such as increased performance, support for SCSI hard drives and CD-ROM devices, easier installation of tools for emulated environments, and support for sound.
I should note at this point that I'm a VMWare user, though that's more complimentary than anything: it's a great product that I use constantly. You can get a 30-day evaluation copy of VMWare 2.0 from the VMWare Web site, and while I don't recommend this product for the casual user, those that do need such a thing should head on over and check it out