W3C Markup Validation Service - Validates HTML markup of web pages, one at a time.
UI Movement - The best UI design inspiration, daily
HTML Validator - HTML Validator is a Mozilla extension that adds HTML validation inside Firefox and Mozilla.
UI8 - Carefully crafted UI design assets
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.
UI Garage - Specific mobile and web design patterns for your inspiration