W3C Markup Validation Service - Validates HTML markup of web pages, one at a time.
Octopus.do - Build your website structure in real-time and rapidly share it to collaborate with your team or clients. Start prototyping websites or apps instantly.
HTML Validator - HTML Validator is a Mozilla extension that adds HTML validation inside Firefox and Mozilla.
PowerMapper - Create visual HTML site maps and XML sitemaps for search engines of any web site quickly and easily.
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.
FlowMapp - FlowMapp is a UX planning tool for creating visual sitemaps and user flow.