Spectre is described as 'A simple UI for browsing and inspecting diffs, and an API for runner scripts to submit screenshots to and receive a pass or fail in real time. (For use with Wraith, Backstop, Selenium etc)'. There are four alternatives to Spectre for Linux, Mac, Online / Web-based, Windows and Node.JS. The best alternative is recheck-web, which is both free and Open Source. Other great apps like Spectre are VisWiz.io (Paid), Applitools Eyes (Paid) and VisualReview (Free, Open Source).
recheck-web is a Golden Master-based test framework on top of Selenium that allows for easy creation and maintenance of more complete and nearly unbreakable tests. If you want to give it a quick try, checkout the Chrome extension that is based on recheck-web.
VisWiz.io is a visual regression testing SaaS tool that helps with catching unwanted UI changes, by integrating with a testing suite and analyzing any visual differences on projects. VisWiz.io supports any UI, from web applications to native mobile and desktop apps.