Website Grader - Grade your website's strengths and weaknesses in seconds.
Beyond Compare - Beyond Compare allows you to compare files and folders.
Checkbot - Checkbot will boost the SEO, speed and security of your site by checking you're following 50+ web best practices. Browser extension with free and paid plans.
WinMerge - WinMerge is an open source differencing and merging tool for Windows.
PageSpeed Ninja - One-click page speed boost for WordPress
Meld - What is Meld? Meld is a visual diff and merge tool targeted at developers.