Among mockup tools I have used over the years, Angle 2 is the most versatile one as Sketch template. Devices colour, material and orientation are all customisable. New update also includes up-to-date devices like iPhone XS, XR and iPad Pro.
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Smartmockups generate ready-to-use product mockups from website link or screenshot. Must-use for creating portfolio or marketing asset.
Perspective mockups could be created without leaving Sketch app. Magic Mirror adds the functionality of perspective transformation to Sketch artboards. So users can create high-fidelity mockups in perspective. Paid monthly subscription support up to 4x image quality and symbols.
Screely turns screenshot into browser mockup with color detection. Free beta right now.