Depending upon your hardware and your needs, VMware offers the ESXi product free of charge. This is a hypervisor based virtual server...meaning that it doesn't require any base os to already be on the box. It loads it's own very stripped down Linux installer that has just the pieces to run the virtual machines.
ESXi handles hardware resources far better than VMWare Server 2.0...however it's quite picky about the type of hardware that it works on. If you are running a genuine server class machine, you are likely just fine. If you are trying to use a cobbled together workstation class machine, you are going to need to check the whitebox sites to see if your hardware is compatible. Here is the whitebox HCL webpage for ESXi4 (ESX / ESXi 4.0 Whitebox HCL