VIOPS VMware Icons and diagrams
Visio stencils made from the artwork as released by the official VMware branding team. Can be used for documentation, now also includes vSphere artwork.
Unfortunately these visio stencils have disappeared, even after asking the VMTN team about this a couple of months ago, they are still gone.
An alternative can be found here: Technodrone: My Visio stencils again (2009)
Update on Maish's his Visio stencils: New vSphere vision stencils (2010)
To be fair, forget the above links, go directly to this one from Maish as it has all of the above stencils as well Technodrone VMware visio stencils
For more alternative visio templates, see the category below for more or search for visio.