Popular stencils
Chemical Engineering 2
A basic stencil for chemical engineering diagrams (Stencil 2).
XING - Interface Elements
User Interface Elements for XING.com - Grids for Header and Home-View - Grids and Elements for boxed elements like "My Contacts"
Arduino
Magnetized shapes for most Arduino boards: Arduino Pro Arduino Mini Arduino Nano Arduino Fio Arduino Micro Arduino Uno R3 and Arduino Leonardo Follow me on Twitter @ngeren
CrossHash
Fill patterns that can be used for process flows.
F5 Networks - BIG-IP - Rack Friendly
BIG-IP 1600 3600 3900 4200 6900 8900 8950 11050 and VIPRION 2400/4400 Dan Strohl's Rack Friendly version
SonicWALL Stencil
This was another Visio stencil that I converted to .gstencil. It includes various SonicWALL security appliances/firewalls and a few miscellaneous networking objects.
Nintendo DS
This is a stencil on a Nintendo DS Lite. Very helpful if you need to do screen mock ups.
Teaser Latest (Magnolia CMS)
Teaser Latest Paragraphs stencil from the Magnolia CMS Standard Templating Kit (STK) Stencil kit.
LOOPS
Use these blocks to make various loop shapes.