Here is the list with some of the most popular visual comparison tools for discovering UI design issues:
LoadFocus Visual Regression Testing – AUTOMATED VISUAL REGRESSION TOOL for Websites and UI Designs
BackstopJS – BackstopJS automates visual regression testing of your responsive web UI by comparing DOM screenshots over time.
Wraith – A responsive screenshot comparison tool
PhantomCSS – Visual/CSS regression testing with PhantomJS
CasperJS – Navigation scripting and testing utility for PhantomJS and SlimerJS http://casperjs.org/
Spectre – 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)
VisualReview – VisualReview, a human-friendly tool for testing and reviewing visual regressions.
Gemini – Gemini is a utility for regression testing the visual appearance of web pages.
WebdriverCSS – Regression testing tool for WebdriverIO
Kobold – Visual regression testing framework, comparing screenshots from multiple builds