I recently had to build an internal network for our training division, enabling the classrooms to be networked and have access to various technologies. Because of the lack of hardware and equipment and running in an exclusive cash only environment I decided to go with a virtual solution on the server.
The server itself is running CentOS 5.1 with VMWare 1.0.6 loaded as the virtualisation provider. On top of this we have 4 Windows Server 2003 machines running, making up the Active Directory, Exchange, ISA, Database and Windows/AV updates component. File sharing and internet routing through the corporate network and ADSL is handled via the CentOS platform.
The setup allows us to expand to physical machines at a later stage quickly, and allows the main server to replaced with minimum downtime on the network, as it only requires the moving of the Virtual Machines and starting them up on the new box.