Popular stencils
Apple Machines
A great stencil with Macintosh and PowerBook centric images.
Mac OS X Stencil
Mac OS X user interface components from the Mac OS X Stencil Kit.
Petri Nets
Petri Nets for modelling synchronization and non-deterministic processes (e.g. operating systems, workflows)
Protocol/Data Block Design
This is a 32-bit ruler and a data field for laying out protocol specification diagrams like commonly used in RFCs and similar publications.
Military Symbols II
This is the second of two stencils of military (NATO) symbols I made and use for my wargames maps and orders of battle. I created it by scanning an existing chart and then converting it into vectors.
Xbox 360 controller
This is a stencil for the Xbox 360 controller. It's very helpful if you need to do game control design.
Endian UTM Appliance
The Endian Firewall product family of unified threat management (UTM) appliances features easy-to-use "all in one" security to deliver maximum protection against Internet threats.
Atoms version 1
A selection of atoms and a bond for use in simple (high school?) chemistry. Atoms are sized according to their actual radii; bonds needs some playing with. The tetrahedral mthane molecule was made by superimposing over a structure drawn with ISIS/Draw (the only reason I need Classic!?)
Grammar
Groupings of text and line or arrow items, styled to nicely represent parse trees, syntax trees, or BNF grammar diagrams.