The online source for product specifications and design files
SEEK can be accessed from most Autodesk programs (Revit, Autocad) just by locating the SEEK bar, typing in your desired information and searching the SEEK database.
Content Creation Sites:
Andekan http://andekan.com/
Broutek http://www.broutek.com/
CadWorks http://www.cadworks.net/
Reed Construction http://www.reedconstructiondata.com/bim/
Digital Building Solutions http://www.digitalbuildingsolutions.com/
4th Dimension http://www.4thdimensionsoftware.com/
London Info Tech http://londonites.com/revit-implementation-services/content-family-creation.html
AligneX http://www.alignex.com/
User Developed Content:
(Be careful with content from these sites as occasionally some content has been a little capricious and corrupted a design file, and you don’t want that to happen in a revenue design file, so test all content in new/blank file first. Still, even poorly designed content is great fodder to reverse engineer and fix up.) :
RevitCity http://www.revitcity.com/index.php
Turbosquid http://www.turbosquid.com/revit