Although you should talk to your production person about the specs of a particular project, there may be times that you need quick sizes to get the design process going. Here are a few quick links to some downloadable templates,

CD-Rom Templates and Packaging,

Templates on Discmakers
Templates on DCA Media Solutions Website

Folders and binders,

Templates on Print Promotions

Outdoor,

Kubin

Direct Mail,

US Postal Service
or
USPS Direct Mail Templates

Various Printed Materials,

Templates on PS Print

Always consult with your production manager or printer before creating your mechanical using these templates, as they may have other template options available directly from the vendor. In the agency’s that I’ve worked in, we keep a set of common templates on the server where everyone can pick them up and utilize them.