Icons8 - Free app for Mac & Windows already containing 39,800 icons. Allows to search and import icons…
Invision - Prototyping and collaboration for design teams
Iconscout - Design Resource Marketplace.
Mockuuups Studio - Fast and easy way to create product mockups on macOS, Windows and Linux.
Craftwork - A collection of User Interface resources made by Craftwork
3dicons - 3dicons library offers beautifully designed open-source 3D icons.