Popular stencils
Tabs
Tabs stencil from the Design Stencil kit in the Yahoo! Design Pattern Library.
Bantik UML
Because UML doesn't have to be black and white... Version 2 turns the 'object' object into a table, making adding an attribute, method, or private method as simple as command-enter.
Rounded Numbers
Why clutter up your diagrams with a lot of text when the most used way of annotating diagrams is by using a number or a letter inside a circle? Very useful when needing to annotate diagrams, drawings, or photographs.
Touchscreen Hand Gestures
A collection of drawn hands for documenting touchscreen interactions. Includes point, tap, pinch, spread, drag.
Web Form Elements
Edward Anastas has put together a handy stencil composed of often-used elements in HTML and web page forms: radio buttons, checkboxes, combo boxes, entry fields and text areas. Nice looking and Aqua, to boot.
Pretty Urban Building Icons
A collection of icons of urban buildings
OS logos
A collection of OS/hardware logos, including Sun, HP, Linux, Red Hat, FreeBSD, Windows, Debian, Darwin, IBM, and VMware.
Web Detail Flowchart
The page title, page file name and page description area labels are editable. I added an example of how I use this stencil. Feel free to use as you like though.
Django Admin
Stencil featuring the out-of-the-box UI elements available when creating admin interfaces for Django sites.