Here's a few Rack Symbols that I've created over the last few years. Some are gear-specific. Some are generic. It works if you work in real-size, i.e. the racks are 19" wide in Omnigraffle, and your grid is 1.75" (a rack space).
This stencil helped me construct an electrical project for my home.
Clef de sol et clef de fa
Wiring diagram blocks for Network Switches from Luminex for use in network AV systems.
iOS UI iPhone stencil from the iPad and iPhone Design bundle.
DEMO is an acronym for 'Design & Engineering Methodology for Organizations'. The name also stands for the demonstration that the underlying PSI-theory (Performance in Social Interaction) is a viable basis for practically dealing with organizational changes of all kinds.
Cisco released updated CVD Icons in many formats, but of course, no Omnigraffle stencil. These are built from the PNGs. http://www.cisco.com/c/en/us/solutions/enterprise/design-zone-smart-business-architecture/cvd.html This is the Wireless Icons Icons section.
Vintage car line sketches.
Model your use-case data as a graph (e.g. Neo4j).