Today, there is various Virtualization Products like VirtualBox, VMware Server/Player/Workstation, Parallels Workstation/Desktop and Windows Virtual PC.
Choice is not always easy, taking consideration of reputation, age, price, press papers, enterprise policies ...
We can find some comparisons, this fresh one is interesting and complete.
VirtualBox has impressively evolved in recent years. In particular with version 2 (released in 2008), and more recently the version 3 (released in june of 2009), it has become an undisputed alternative to other solutions.
Nowadays, we can read things like "With support for up to 32 virtual CPUs per VM, VirtualBox is now the class leader in terms of raw virtualization muscle", and "VirtualBox is now poised to challenge VMware and Microsoft in the datacenter" (source).
VirtualBox Wikipedia article is interesting too.
The list of features is impressive, and the hyper-reactivity (see the changeLog) makes it a very powerful product.
In addition, the integration to various GNU/Linux distribution like Fedora is a very important point making it very easy to install and upgrade the product.
For instance, for these two last points, that is not the case of the free version of VMWare server; its modules' source code has not evolved since months, so it must be patched to be used with newer kernel versions (see this article).
Today, with the maturity of the version 3 (features, powerfulness, hosts/guests support, integration), and in addition because it is an open-source product, and free of charge, VirtualBox has become some kind of obvious choice.