Some links to free 3D models, that are compatible (.blend file, or can be imported) with Blender:

The Official Blender Model Repository
http://e2-productions.com/repository/index.php

Blender Architecture related 3D models:
http://blender-archi.tuxfamily.org/Models 
http://resources.blogscopia.com/index_en.html

Cars:
http://dmi.chez-alice.fr/models1b.html

Sci-Fi (starwars, star trek, etc):
http://www.blenderwars.com/content.php?module=download
http://www.blenderwho.co.uk/

3D Models of Various Formats, some of which can be imported into Blender
http://telias.free.fr/Models_menu.html

Motion Capture
http://mocap.cs.cmu.edu/
http://feeblemind.tuxfamily.org/dotclear/index.php/2007/08/17/94-around-600-mocapad-data-sets-on-mocapdatacom