Originally a commercial product VirtualBox is now open source. The open source version lacks only few features. Having said that VirtualBox is still a great virtualization tool and future versions look very promising. Like all other virtualization software VirtualBox allows you to run a guest operating systems on your PC, in this case either Windows or Linux based. The guest operating system support covers most Windows (including Vista) and Linux versions as well as OS/2 Warp, OpenBSD and FreeBSD.
VirtualBox is a stable tool with performance similar to it's commercial counter pieces. A product well worth trying out
Xen is state-of-the-art virtualization on the PC - and it is even open source. With a high performance mainly due to it's architecture with the hypervisor and it's support for hardware virtualization - Xen sets a new standard for virtualization software. Already used in many data centers Xen has shown that it is stable and a product you can rely upon. It matches almost any feature you get from commercial virtualization software - but includes the most advanced such as virtual machine relocation and per virtual machine resource guarantees.
OpenVZ is a great alternative to standard virtualization offerings on the market, that unlike most other products only offers operating system level virtualization - and only on Linux. However, this is also it's strength - but no providing a complete virtual pc to run any operating system, OpenVZ's shared operating system provides better utilization of your server hardware.
Every virtual server running under OpenVZ has full root access, but OpenVZ makes sure that there are no conflicts between the different virtual server. Thus, each server has it's own IP addresses, dedicated memory, application, libraries and so on.
OpenVZ is backed by SWsoft and could be considered an open source edition of the Virtuozzo product.
coLinux is an extremely interesting new approach to virtualization allowing you to run Linux parallel to your Windows platform. coLinux approaches the experimenting Linux novice - who does not actually want to install the operating system on a fresh machine. Another target group is the Linux enthusiast, in that coLinux also makes it possible for him to run Linux on his Windows machine without using a standard virtualization product with large requirements to system resources.
coLinux seems to be a promising ongoing project - and a worthy competitor to other virtualization products on the market. It might not be completely fool proof for beginners, but it may become more so as it matures.