Open Peeps - A hand-drawn illustration library.
Mockuuups Studio - Fast and easy way to create product mockups on macOS, Windows and Linux.
Control Illustrations - 108 free flat illustrations with customizable characters
Mediamodifier - Create beautiful designs and product mockups in seconds.
Craftwork - A collection of User Interface resources made by Craftwork
mockDrop - Is a webapp for creating free device mockups including your screenshots.