Turbosquid | Free models: A reference. A large collection of free models and a huge library of paid models.
GreyscaleGorilla | Free model pack: Some cool free models by the GreyscaleGorilla Team.
The Pixel Lab | Freebies: Just subscribe their newsletter to get access to their nice model library.
Archive 3D | 3D Furniture: If you need furniture models, this is the place.
Lincoln 3D Scans: 3D scans of objects from the Usher Gallery and The Collection in Lincoln.
British Museum Scans: 3D scans of objects from the British Museum
Smithsonian 3D Scan: Miscellaneous scans
Artec3d: Even more scans!
The Mantissa | Resources: Award-winning CG Artist shares some of his really good resources.
The French Monkey: Constantin Paschou is also a 3D artist who shares lots of great freebies.
Aaron Covrett: Aaron is motion designer at Stink with great resources to share.
XOIO: A little bit of everything on this German website, they have great 3D scanned people.
NASA 3D models: 3D resources from the NASA! From spaceship to satellite — you can literally recreate space!
Google | Poly: The free library by Google.
Sketchup Warehouse: Contains millions of models created in Sketchup, but you can usually use them with other software, too.
Note that when you download model from web, the file format is not always the one you need. Here is a great online tool to help you transform it to the one you need: http://www.greentoken.de/onlineconv/