BlackBee 3D
A site where you can buy a few nice revit family models for a reasonable price. They have pillows, towels, and plants, which is great for rendering.
Revit Families
Archiproducts
My second go-to site for Revit Families. It focuses more on interior design than the other resources (large furniture and lighting repository). The only issue is that they "cut you off" after so many daily downloads. Also, lots of brands that don't open their BIM files on Archiproducts will have files on their own sites (so you can either request through Archiproducts or click through to the vendor website to look).
RevitCity.com
Community-generated content. Great for generic stuff and accessories that you need for renderings and scenes, but not so much for brand stuff...plus quality is varied!
BIMsmith Market
Excellent resource library for BIM files as well as 3-part specifications. They have a nice selection of doors on the platform.
BIM&CO
Excellent place for downloading Revit files, especially regarding appliances, HVAC, and other equipment. Smeg, panasonic, hitachi, samsung, bosch, etc. - organized by CSI Divisions/Masterformat
ARCAT
Lots of building products. You'll find 3-part specifications, CAD, Revit, etc, but it's best for construction stuff - profiles, soffits, solar panels, industrial doors, etc. - organized nicely by CSI Divisions/Masterformat
bimstore
Has some more obscure brand revit families (Gresham?) and user-generated content. Some fun objects (including holiday decor and toys)
BIMobject
The place where you'll find the most RFA/RVT files for vendors anywhere. Free. Amazing resource. (Absorbed Polantis as well)
Configura
Some brands will share their 3D models here (and use this as their configurator), but you need to contact them to get added to their "briefcases." Generally, you can access these on their websites. The files to download aren't always Revit.
NBS National BIM Library
More focused on building materials, but you can find doors, windows, hardware, etc.
Syncronia
Italian-based upscale furniture search with many free 3D downloads for furniture, accessories, lighting, doors, plumbing fixtures, and more.
MyResourceLibrary
Some brands will share 3D models here. You can sign up for a free account and browse through their catalogue of vendors. Most have Revit and AutoCad files as well as spec sheets.