VirtualBox OSDSE is the open source/free x86 virtualization solution allowing a wide range of x86 operating systems such as Windows, DOS, BSD or Linux to run virtually on a Linux based system. Great for office environments and gamers.
The free VMware Server lets you run pre-built virtual machines on your desktop. You can run multiple operating systems side-by-side, easing the process of software development, testing, and evaluation. VMWare also requires specified kernel modules which will also be installed or may be built from source.
Qemu Launcher provides a graphical front-end to all basic, and many advanced, QEMU computer emulator options. It allows you to create, save, and run multiple virtual machine configurations, create and also convert disk images. Great for testing out other operating systems or running them in virtualization within your own.
Wine allows you to run thousands of Windows applications under Linux including popular titles like Adobe Photoshop, Internet Explorer, and World of Warcraft. Usually thought of as a Microsoft Windows emulator, the Wine developers would prefer that users thought of Wine as a Windows compatibility layer for Linux.