My good colleague Oliver just linked me to this awesome wirefaming stencil kit from Yahoo!. These stencils contain just about every common element you'd need for wireframing anything from websites to iPhone apps. They even provide all kinds of different formats (Visio, PDF, SVG...to name a few).
Here's a list of the different stencils located in the zip file:
Here's some direct download links:
OmniGraffle
Visio (XML)
PDF
PNG
SVG