Optimizely - A/B testing you'll actually use.
Monkey Test It - Primate powered algorithms that scan your site for common bugs and SEO mistakes.
Google Analytics - Improve your website to increase conversions, improve the user experience, and make more money using Google Analytics. Measure, understand and quantify engagement on your site with customized and in-depth reports.
UserTest.io - User testing made simple. High-quality, affordable user testing solutions from real users, here in the UK. Improve your usability!
VWO - Use VWO to tweak, optimize & personalize your website with minimal IT help.
PreFlight - Preflight is an Automated Web Testing tool. Allows everyone to automate Web UI tests.