VMware virtualization software lets you run one operating system within another. This is quite useful for security researchers who commonly need to test code, exploits, etc on multiple platforms. It only runs on Windows and Linux as the host OS, but pretty much any x86 or x86_64 OS will run inside the virtualized environment. It is also useful for setting up sandboxes. You can browse from within a VMware window so the even if you are infected with malware, it cannot reach your host OS. And recovering the guest OS is as simple as loading a "snapshot" from prior to the infection. VMware player (executes, but can't create OS images) and VMWare Server (partitions a physical server machine into multiple virtual machines) were recently released for free. An open-source alternative is VirtualBox. Xen is a Linux-specific virtualization system.
For downloads and more information,
visit the VMware homepage.