Popular stencils
Flat TV
A simple flat TV
iOS Wireframe iPhone
iOS Wireframe iPhone stencil from the iPad and iPhone Design bundle.
Essence Kernel
This is a stencil for the Essence Kernel framework.
Cisco CVD 2014 LAN Switching Icons and Clouds
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 LAN Switching Icons and Clouds section.
Sinusoids
A nice combination of sinusoid shapes (derived from trigonometric functions).
ShoreTel - ShoreLine/ShoreGear VOIP stencil
This is a Visio stencil that I had to export in Visio and Save As an XML stencil. I then converted that in OmniGraffle Pro to a .gstencil. Stencil contains a few Shoreline and Shoregear voice and T1 switches, various ShoreTel phones and generic buildings.
Diagrams for categories
Basics for making mathematical diagrams in category theory, abstract algebra, etc.
Apple Final Cut Pro X icons (2015)
Includes most of the major Final Cut Pro X icons (with Magnets), updated for FCPX v.10.2. Also included are updated Compressor, Motion and some Third Party apps. Handy for designing workflows.
Java App Server Logos
These logos/icons were made for use in diagraming enterprise java application environments with non-heterogenous architectures.