If you want to try out some cluster configurations, VMWare server has a nice feature where you can share a vmdk between more than one VM, and since it support SCSI persistent reservations, you can truly simulate a real cluster. I have used this to deploy Redhat Cluster server as a test environment.

Based on the documentation on the VMWare website, sharing a disk between multiple nodes can be achieved by creating a virtual disk, and then importing that disk into each virtual machine.
Once the disk is imported, you can add or modify the scsiX.sharedBus = "virtual" (X should match the controller that is used to host the shared disk) and disk.locking = "false" directives to the shared disk stanzas in each virtual machines vmx file.
Here is a sample:

scsi1.present = "TRUE"
scsi1.sharedBus = "virtual"
scsi1.virtualDev = "lsilogic"
scsi1:0.present = "TRUE"
scsi1:0.fileName = "shared_disk.vmdk"
disk.locking = "false"

This has served me well for testing and evaluation purposes for quite some time, though I provide ZERO guarantees that it will work in yours (and there is always the possibility that sharing disk between multiple nodes will corrupt data). I am documenting this here as a reference for myself, though feel free to use this information at your own risk.