Clay Mockups - Free set of beautiful device mockups for your designs
Invision - Prototyping and collaboration for design teams
Craftwork - A collection of User Interface resources made by Craftwork
Mockuuups Studio - Fast and easy way to create product mockups on macOS, Windows and Linux.
AppLaunchpad - Create free beautiful screenshots for App store/Google play
Ape Mockups - A huge hand-curated collection of free high-quality mockups