I am frequently creating Visio diagrams to use for software development or web applications. I typically use the built in elements and icons. Recently I have found it difficult to find some items that I would like to have in my designs. The great team and group over at OSA (www.opensecurityarchitecture.org) have many tools to both help with designs but also elements in the designs.
If you are not familiar with OSA I would recommend looking at some of their patterns or their library to understand how this great group can be useful. They help both visually display IT standards but also security standards.
Recently I have been using their Creative Commons released Icon set to help in my designs. Simple icons like padlocks, wifi signals, users, and servers. Not only are they good looking but it is very functional. It never hurts to have a design that is both elegant and useful.