These are a few of the model resources I used to use when working with 3D in the past.
3D Links is a great resource for everything with 3d. It has a long list of links to sites offerering 3d objects.
Another source I used a lot to get my interior models from is e-interiors. You have to do a search for 3D in the Cad Library Availablity. It has hundreds of free interior objects from chairs to sinks.
For smaller assemblies and parts like knobs an gears have a look at 3D content central
Exchange 3D also has a list of free models
If you want 3D diamond cuts for experiments try 3D Lapidary
KatsBits has a couple of low res md5 game models
Last I want to mention De Espona. This collection is not free (except for a few models, available here). It costs $395 for the downloadable version. But, if you're looking for an entire encyclopedia with models in various resolutions, I would recommend this one.