Invision - Prototyping and collaboration for design teams
3dicons - 3dicons library offers beautifully designed open-source 3D icons.
Mockuuups Studio - Fast and easy way to create product mockups on macOS, Windows and Linux.
Wannathis - 3d illustrations and freebies
Craftwork - A collection of User Interface resources made by Craftwork
IconShock - Over 2 million free icons in 400+ icon sets over 30 styles