The combo of open source and virtual machines is really providing some powerful options for rapid evaluation and prototyping.
Here is an example ... "Want to do some network management?" ... then download the $10 PDF of "Network Monitoring with Nagios", then get the VMWARE Player, and then check out the Nagios appliance deployment for VMWARE.
In a couple of clicks, your up and running and ready to do some network management. If you like what you see, a couple of file transfers, and some more clicks, and you can be in production mode. Nice.
Thanks to a recent HomeNetworkHelp.info podcast for getting my thoughts moving in this direction.