Ape Mockups - A huge hand-curated collection of free high-quality mockups
Now UI Kit - A beautiful Bootstrap 4 UI kit. Yours free.
AppLaunchpad - Create free beautiful screenshots for App store/Google play
Bots UI Kit - Fully customizable Sketch UI Kit for Messenger Platform
Clay Mockups - Free set of beautiful device mockups for your designs
Landing UI Kit - Handcrafted UI components for landing pages