I've used VMWARE ESX / Xen paravirtualization, Virtuozzo, Solaris Containers, and FreeBSD Jails as os level virtualization. Virutalbox is another full virtualization solution. Presently, VirtualBox runs on Windows, Linux and Macintosh hosts and supports a large number of guest operating systems including but not limited to Windows (NT 4.0, 2000, XP, Server 2003, Vista), DOS/Windows 3.x, Linux (2.4 and 2.6), and OpenBSD.
Rakesh has published a small article about VirtualBox Virtualization software. Both Windows and Linux can be run together simultaneously, and you don't even need to switch between the two. With the seamless Windows feature of the latest version of VirtualBox virtualization software, you can seamlesssly run both Windows and Linux applications from the same desktop interface. This has been made possible by the combined efforts of VirtualBox and SeamlessRDP that is meant for seamless Windows support for rdesktop.
=> How to run Windows and Linux at one place? [ciol.com]