Software Testing Social Network

Site Check
Type in one URL and automatically run HTML and stylesheet validators, accessibility assessment, link check, load time check, and more. Organizes access to a collection of free online web test tools. Site of UITest.com/Jens Meiert. Also lists a wide variety of free online web analysis/development/test tools.
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Dead-Links.com
Free link-checker limited to 25 pages per domain and 150 external documents. Higher limits if site has a link to Dead-Links.com.

WDG HTML Validator
Web Design Group's validator - latest HTML version support, flexible input methods, user-friendly error messages.

Web Page 'Purifier'
Free on-the-web HTML checker by DJ Delorie allows viewing a page 'purified' to HTML 2.0, HTML 3.2, HTML 4.0, or WebTV 1.1. standards.

W3C HTML Validation Service
HTML validation site run by the WWW Consortium (the folks who set web standards); handles one URL at a time; Can choose from among 30 character encoding types, and multiple HTML and XHTML document types/versions.

W3C CSS Validation Service
CSS validation site run by the WWW Consortium (the folks who set web standards); handles one URI at a time; or upload file or validate by direct input.

W3C Link Checker
Link checking service run by the WWW Consortium (the folks who set web standards); configurable. Handles one URL at a time. PERL source also available for download.
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NetMechanic
Link checker, HTML validator, Meta Tag/keyword analysis from Keynote Systems. Type in the site URL to check. Free and fee-based.

Bobby
Free service from Watchfire to help web page authors identify and repair significant barriers to access by individuals with disabilities. Can test one page at a time against. Can test against multiple standards simultaneously. Tests against W3C-WAI guidelines and US Section 508 guidelines. Downloadable version available for purchase; for Windows.

Doctor HTML
Site with online web page checker by Imagiware. Checks spelling, forms, table structure, form structure, tag usage. Validates links. Free to analyze single page. Use paid SiteDoctor service to analyze entire sites. Downloadable version also available for purchase; runs on multiple OS's.

Weblint Gateway
Site with online HTML validator; somewhat configurable.

Web Page Backward Compatibility Viewer
On-the-web HTML checker by DJ Delorie; will serve a web page to you with various selectable tags switched on or off; very large selection of browser types; to check how various browsers or versions might see a page.