vizioncore published a nice guide to use VMware ESX Server with a NFS remote storage hosted on Microsoft Windows.
Read it here.
While this method provides a very low cost solution for ESX Server testing environments, it still implies the availability of a Windows license.
To cut away this cost as well I strongly recommend considering 2 open source projects:
Both allow to create a fully working network storage from bare metal, and without the hassle of configuring a complete Linux distribution.