3D Content Central is a resource for free 3D and 2D CAD models of user-contributed and supplier-certified parts and assemblies. The service is operated by Dassault Systemes, makes of SolidWorks, and as such is a bit more business and industry focused than most other 3D libraries. The models are organized into hundreds of categories, and are optimized for use in SolidWorks. This is a good source if you're looking for a model of a particular part that is made by a particular company, or if you're looking for assembly models to use in a larger product model.