Popular stencils
littleBits components
Stencil of littleBits (http://littlebits.cc) components.
CTERA Networks
CTERA Networks CloudPlug, C200, C400, & Portal.
Symbole für Flucht- und Rettungsplan / escape plan
Symbole für Flucht- und Rettungsplan escape plan
Direct Logic 06: Chassis
These are the PLC micro chassis: DL06. The symbols are similar, but each has a different silkscreen.
Red Meat
Various cuts of meat for butchery diagrams and other uses. Hamburgers, steak, etc.
A/V Switching Components (Incl. Spyder, Encore, SP2)
This is a collection of common audio visual switching components including the Vista Spyder, Screen Pro II, and Encore.
AWS Deployment Icons
AWS Simple Icons for Architecture Diagrams from http://aws.amazon.com/architecture/icons/ converted to stencils.
Audio Rack Symbols
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).
HP ProLiant 3D
HP ProLiant Servers