W3C Markup Validation Service - Validates HTML markup of web pages, one at a time.
Netumo - Ensure healthy website performance, uptime, and free from vulnerabilities. Automatic checks for SSL Certificates, domains and monitor issues with your websites all from one console and get instant notifications on any issues.
Total Validator - Total Validator can check your website to help ensure that it is accessible, uses valid HTML and CSS, has no broken links, and is free from spelling mistakes.
AppZero - AppZero is a monitoring and migration tool that allows users to keep track of different applications and servers in both simple and complex IT environments.
Rocket Validator - Site-wide Accessibility and HTML validation made easy. With a single click, Rocket Validator will check thousands of web pages on your site for Accessibility and HTML conformance, producing a detailed, actionable report on seconds.
RedGate SQL Monitor - SQL Monitor helps you and your team find issues – before they become problems