Note: These are only the ones that I recommend. Not all validation software is made equal.
[Progress: I am still compiling this list from my scribbles, bookmarks, post-it notes, and random note files. It is a work in progress. – Last updated on April 9, 2006.]
(X)HTML and XML Validators
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W3C Online HTML & XHTML Validator
Checks (X)HTML, SVG, and MathML web documents for conformance to W3C Recommendations and other standards. This can be used for both online files, and uploaded offline files.
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WDG (X)HTML Online Validator
Checks HTML and XHTML documents in the same way that the W3C validator does, but it includes an option to spider the entire site and check all your pages at once, as well as offering a batch mode which will check multiple urls.
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W3C Online XML Schema Validator
Checks your XML schema for compliance with the most recent recommendations.
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STG Online XML Validator
Checks your XML documents for compliance.
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W3C Online RDF/XML Validator
Checks your RDF/XML documents against the most recent supported recommendations.
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Others
You may see a list of others here at DMOZ and here at WDG.
CSS Validators
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W3C Online CSS Validator
Checks CSS in (X)HTML documents or standalone for conformance to W3C recommendations.
RSS/ATOM Feed Validator
Accessibility Compliance Checks
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Bobby – Watchfire WebXACT
One of the best online accessibility compliance checks in existance. Can check 508 compliance, and all levels (A, AA, AAA) of the WCAG guidelines.
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Cynthia Says
A favorite of mine for the same sort of quality checks you get from Bobby, with less interface clutter.
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WAVE
Powered by the folks over at the marvelous WebAIM, this is a very good accessibility checker.
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TAW
TAW (Web Accessibility Test – backwards) is another online check using the WCAG guidelines.
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HiSoftware
Section 508 and WCAG 1.0 Check.
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Site Valet
Section 508 and WCAG 1.0 Check.
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BITV
Section 508 and WCAG 2.0 Check.
Online Shopping
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eBay sniper
A tool to automatically bid on auctions.