http://www.tracepartsonline.net (Very good)
http://www.3dcontentcentral.com/default.aspx
http://www.thomasnet.com/cadmodels.html
http://portal-en.partcommunity.com/
http://www.hongkiat.com/blog/60-excellent-free-3d-model-websites/
http://www.hongkiat.com/blog/25-free-3d-modelling-applications-you-should-not-miss/
Keyword to search :
free 3D parts -> for mechanical
free 3D models -> for stuffs
