The best alternatives to frameshift are Smartmockups, Mock, and Browser Mockups. If these 3 options don't work for you, we've listed a few more alternatives below.
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...
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 ...