Popular stencils
Wireframing stencil
Created by Brian Talbot. For more info, check out www.brian-talbot.com.
Simple D&D Map Builder
This is a simple, black and white, stencil for drawing D&D Maps. Create a new document and set it's gridlines on and ensure it is set to 1 inch or 2.54 cm. set gridlines to print. Then, you can drag over the elements of a dungeon. Let me know what you think via my website
Event-driven Process Chains
Event-driven Process Chains (EPC) are used widely for describing business logic within a company. EPCs have been invented by Prof. Scheer and commercialized by IDS-Scheer: http://www.ids-scheer.com/international/en
Maps
While OmniGraffle probably isn't a tool for professional cartographers, we've found it can be quite useful for making informal maps and directions.
Box-and-Arrow Diagrams
The classic elements of box-and-arrow diagrams, widely used in describing Lisp data structures constructed from cons cells.
Formal Network Symbols
This is an excellent stencil set of formal network symbols, for when the poofy internet cloud simply isn't enough. Oh wait — the poofy internet cloud is still there, hiding behind the moniker of "Network Cloud". That seems formal, maybe not black tie, but pretty formal.
Napkin UML
Simple hand-drawn UML stencil.
Cables
Cables of computer network and digital devices.
Floorplans: Furniture
A handy collection of furniture for laying out basic floorplans.