Popular stencils
Wireframing stencil
Created by Brian Talbot. For more info, check out www.brian-talbot.com.
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.
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
Cables
Cables of computer network and digital devices.
Box-and-Arrow Diagrams
The classic elements of box-and-arrow diagrams, widely used in describing Lisp data structures constructed from cons cells.
Floorplans: Furniture
A handy collection of furniture for laying out basic floorplans.
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.