Smartmockups - Create stunning product mockups, easily and online.
Doka.Photo - Edit images without leaving your browser
AppLaunchpad - Create free beautiful screenshots for App store/Google play
Sumo Paint - Photoshop in your browser, Sumo Paint an advanced browser-based image editor.
Mockuuups Studio - Fast and easy way to create product mockups on macOS, Windows and Linux.
Really Good UX - A library of screenshots for UX inspiration