UML Deployment Diagram Nodes (the 3D blocks) have a very usefull feature I find: Tags. It allows showing the OS type, or some other requirements of a node. Example: {OS=Windows x86}.
Diagram example on http://www.agilemodeling.com/artifacts/deploymentDiagram.htm
I found tags in the properties of a Node in VP-UML, but no where to show those tags on the diagram.
