The best alternatives to frameshift are Smartmockups, Mock, and threed.io. If these 3 options don't work for you, we've listed a few more alternatives below.
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threed.io is a browser based tool to generate custom device mockups on the fly. Upload your Designs, make adjustments and download your custom mockup! Even works pretty nice on mobile. We're still in beta, but if you're interested in threed, please read the about text on the site.
Browser Mockups is a tool for creating 100% CSS browser mockups that you can copy and paste into your web page. Our simple interface lets you select from a wide range of configurations to generate a mockup that fits your branding in seconds.