TurboSquid is a popular site for 3D content that is branching out into the Revit Family realm. In partnership with Autodesk, they just opened up their "Revit Market", a marketplace especially for user-generated Revit families. "Revit Market" is designed to complement Autodesk Seek.
To encourage users to publish Revit families for sale, TurboSquid is holding a Revit Contest with cash prizes up to $2500. Deadline is Jan. 31, 2009.
A little bit about them clipped from their site:
The largest library of 3D products for sale in the world.
A clearinghouse for digital artists to make money selling their content.
A place to find premium plug-ins for 3D software applications.
A forum for 3D professionals to exchange ideas.
A service accessible by a traditional website as well as our free, downloadable client applications for PC and Mac.
A production tool to help digital artists more rapidly find, preview and acquire essential content.
The TurboSquid Mission:
Our goal is to revolutionize the way that 3D products are bought, sold and delivered by creating a new and focused digital marketplace designed to offer large selections of affordable products while providing maximum returns to intellectual property owners. We strive to provide unmatched value and support to our partners, content creators and customers while creating a stimulating and exciting environment to accomplish our plans.
EDIT: They've started a blog called Revit Market.