HTML/XHTML Validator

  o The W3C Markup Validation Service
    HTML/XHTML Validator の定番
    http://validator.w3.org/

  o Another HTML-lint
    http://openlab.ring.gr.jp/k16/htmllint/

  o WDG HTML Validator
    http://www.htmlhelp.com/tools/validator/

CSS Validator

  o W3C CSS 検証サービス
    CSS Validator の定番
    http://jigsaw.w3.org/css-validator/

アクセシビリティ

  o Watchfire WebXACT
    http://webxact.watchfire.com/

RDF/XML Validator

  o W3C RDF Validation Service
    http://www.w3.org/RDF/Validator/

  o FoaF Explorer
    http://xml.mfd-consult.dk/foaf/explorer/

  o ccValidator
    http://www.yergler.net/projects/ccvalidator/

RSS Validator

  o Feed Validator for Atom and RSS
    http://feedvalidator.org/

  o Redland RSS 1.0 Validator and Viewer
    http://librdf.org/rss/

Reference

ValidatorCollection - HTML/XML 検証サービス一覧
http://hpcgi3.nifty.com/spiegel/?ValidatorCollection