VirtualBox is a general-purpose full virtualizer for x86 hardware. Targeted at server, desktop and embedded use, it is now the only professional-quality virtualization solution that is also Open Source Software.
Some of the features of VirtualBox are:
Modularity. VirtualBox has an extremely modular design with well-defined internal programming interfaces and a client/server design. This makes it easy to control it from several interfaces at once: for example, you can start a virtual machine in a typical virtual machine GUI and then control that machine from the command line, or possibly remotely. VirtualBox also comes with a full Software Development Kit: even though it is Open Source Software, you don't have to hack the source to write a new interface for VirtualBox.
Virtual machine descriptions in XML. The configuration settings of virtual machines are stored entirely in XML and are independent of the local machines. Virtual machine definitions can therefore easily be ported to other computers.