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