Virtual machines allow you to run one operating system emulated within another operating system. Your primary OS can be Windows 7 64-bit, for example, but with enough memory and processing power, you can run Ubuntu and OS X side-by-side within it.
VirtualBox (Windows/Mac/Linux, Free)
Parallels (Windows/Mac/Linux, $US79.99)
VMware (Windows/Linux, Basic: Free, Premium: $US189)
VMware for desktop users comes in two primary flavors: VMware Player and VMware Workstation. VMware Player is a free solution aimed at casual users who need to create and run virtual machines but don't need advanced enterprise-level solutions. VMware Workstation includes all the features of VMWare Player—easy virtual machine creation, hardware optimisation, driver-less guest OS printing—and adds in the ability to clone machines, take multiple snapshots of the guest OS, and a replay changes made to the guest OS for testing software and recording the results within the virtual machine. You can read more about VMware Player here and VMware Workstation here.
QEMU (Linux, Free)
QEMU is a powerful virtualisation tool for Linux machines built upon the back of the KVM system (Kernel-based Virtual Machine). QEMU executes guest code directly on the host hardware, can emulate machines across hardware types with dynamic translation, and supports auto-resizing virtual disks. Where QEMU really shines, especially among those who like the push the limits of virtualisation and take their virtual machines with them, is running on hosts without administrative privileges. Unlike nearly every emulator out there QEMU does not require admin access to run, making it a perfect candidate for building thumb-drive based portable virtual machines.
Windows Virtual PC (Windows, Free)
Compared to the other any-OS-under-the-sun virtual machine applications in this week's Hive Five, Windows Virtual PC is a tame offering. Windows Virtual PC exists solely to emulate other—usually earlier—versions of Windows. If you need to run an app that only works under Windows XP or test software for backwards compatibility with Vista, Windows Virtual Machine has you covered. It's limited, true, but for people working in a strictly Windows environment—and most of the world still is—it gets the job done. Note: Virtual PC is availabls as Virtual PC 2004, Virtual PC 2007, and Windows Virtual PC, use this host and guest OS compatibility chart to figure out which one fits your needs.
Want to share a great feature of your favourite virtual machine app? Let's hear it in the comments.