Datadog - See metrics from all of your apps, tools & services in one place with Datadog's cloud monitoring as a service solution. Try it for free.
Website Grader - Grade your website's strengths and weaknesses in seconds.
AppDynamics - Get real-time insight from your apps using Application Performance Management—how they’re being used, how they’re performing, where they need help.
PageSpeed Ninja - One-click page speed boost for WordPress
Zabbix - Track, record, alert and visualize performance and availability of IT resources
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.