Optimizely - A/B testing you'll actually use.
Smaller - Make your websites load faster
108 Proven Split Test Winners - Simple tweaks to make to your website, to make more money!
Website Grader - Grade your website's strengths and weaknesses in seconds.
Growth Book - The Open Source A/B Testing Platform
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.