Generic Content
http://www.usgdesignstudio.com/
http://revitcomponents.blogspot.com/
http://www.revitfamilyman.com/
http://www.productspec.net/cad-files.aspx
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Specialized Content
http://www.acuitybrandslighting.com/
http://www.americanstandard-us.com/
http://www.focalpointlights.com/
http://www.usgdesignstudio.com/
http://www.woodworkinstitute.com/
http://www.bradleycorp.com/bim/products/bim/
http://www.polantis.com/la-toulousaine
http://www.coalesse.com/resources.html
http://www.arcxl.com/architects/
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Materials
http://www.nora.com/us/resources/design-tools/revit-materials-library/
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Software
http://smartbim.reedconstructiondata.com/
http://www.reedconstructiondata.com/bim/bim-object-creation/
And more here