Andrew Kator is hosting an impressive array of ready to use Blender objects on his Blender 3D Model Repository ranging from vehicles to animals, architecture and electronics. Most of the models are released under the Blender Artistic License.
Update [2006/11/25]: The site no longer exists; read more about it and an alternative.
The model repository was created to help with all of the dead links that are *everywhere* in the Blender community. So far it is working very well at that task. Since started in June, there are over a million model downloads.
I did a quick count and found over 500 models, most of which are of good quality.