W3C Web Quality Tools
Web Quality Tools – web-based, free, opensource, online or downloadable tools, developed and hosted by W3C.
UNICORN – Improve the quality of the Web
„Improve the quality of the Web“

https://validator.w3.org/unicorn/
Gathers the results of the HTML and CSS validators, as well as other useful services (Internationalization, RSS/Atom feeds, http headers).
MarkUp Validator
„Check the markup (HTML, XHTML, …) of Web documents“

https://validator.w3.org/
Checks the markup validity of Web documents in HTML, XHTML, SMIL, MathML, etc.
CSS validator
„Check Cascading Style Sheets (CSS) and (X)HTML documents with style sheets“

https://jigsaw.w3.org/css-validator/
Checks Cascading Style Sheets (CSS) and (X)HTML documents that use CSS stylesheets.
Link Checker
„Check links and anchors in Web pages or full Web sites“

https://validator.w3.org/checklink
Looks for issues in links, anchors and referenced objects in a Web page, CSS style sheet, or recursively on a whole Web site.
mobileOK Checker
„Is your Web site mobile-friendly?“

https://validator.w3.org/mobile/
Log Validator
http://www.w3.org/QA/Tools/LogValidator/