Cynthia Says

Check a given web page for Section 508 standards problems and other accessibility issues. The free service allows for single-page checking, and is rate-limited to prevent abuse.

Doctor HTML Single-Page Analysis

Performs a wide range of tests on a given page, including document structure, spell checking, cross-browser support, a link check, HTML errors, META tag checks and more. A premium version of the tool offers site-wide verification with additional reporting.

HTTP / HTTPS Header Check

Inspect the HTML headers being returned by a given web server when it returns a URL. Useful for instance to check whether 301 redirects are working correctly, and to make sure that 302 redirects are not being served up instead.

HTTP Status Codes Checker

Server header inspection tool that works in single URL and batch modes. Can detect all the primary HTTP status codes (200, 301, 302, 304, 307, 404 and 410) and follow redirects to show the before-and-after status codes.

mod_gzip test

Test to see whether a specific URL (site or page) is being compressed with the Apache mod_gzip module, and also whether a particular browser is able to accept gzipped content compressed in that way.

NetMechanic HTML Toolbox

The free sample version of the HTML toolbox returns a limited amount of information about up to 5 URLs, including HTML problems, browser load times, browser incompatibility problems and other issues. Premium subscribers have access to additional reporting and can verify a wider range of issues.

Robots.txt Checker

This tool verifies the consistency and validity of your robots.txt file, to ensure that search engine spiders will behave as expected on encountering it.

Screen Size Tester

Free tool lets you see how your site will look at various standard screen resolutions. Note that since the tool actually resizes your browser window to the appropriate size to frame the content, your system has support at least the resolution you want to test!

Vischeck

Vischeck produces a simulated snapshot of your webpage as a color blind visitor might see it. You can select from 3 types of color blindness problems, with screenshots generated accordingly.

W3C CSS Validation Service

Check the validity of your CSS code. You can upload the code to be checked, paste it into a text box or check a remote URL. You can validate against different CSS levels using the advanced interface.

W3C Markup Validator

Free online tool to check the validity of HTML and XHTML documents. You can validate a given URL, or upload a file for checking. Various reporting options provide additional information about warnings and errors.

WebXACT

Free online tool to analyze a given page for quality, accessibility and privacy issues. The basic free tool provides a substantial amount of information; a premium corporate-level tool is available for even more in-depth checking and reporting.

Worth a look if you're willing to pay...