Shapefest - A massive library of 3D shapes for everything!
Interfacer - Collection of more than 200+ free design resources
Wannathis - 3d illustrations and freebies
Facebook Design Resources - A collection of free resources made by designers at Facebook
LostGeometry.xyz - Huge set of 91 abstract 3D objects in various styles.
Freebiesbug - Collection of the best free web design resources.