Free HTML / CSS / Link Validation

In a nutshell: this page lists HTML validators (which check HTML code for standards compliance and errors), CSS validators (which check Cascading Style Sheets for errors and standards compliance), browser compatibility checkers (checks your page to see if it is compatible with various browsers), broken link (or dead links) checkers, accessibility checking (check if your web page is accessible to people with disabilities), etc.

Free HTML Validators, XHTML Validators, Browser Compatibility Checkers

W3C HTML Validation Service: You can use the W3 Consortium's HTML validator to check your web documents either by giving it your web page's URL or uploading your document directly to the validator. Many people consider it the authoritative source for checking your web pages for standards compliance (since the W3 Consortium is responsible for the various HTML/CSS standards). Source code for the validator is also available.
HTML Tidy (HTML Validator): HTML Tidy is not just a HTML validator for your web documents. It is able to correct (repair and fix) bad HTML code and give you pointers to areas where you need to change in order to make your web page accessible to people with disabilities. Source code and binaries are available for a wide variety of operating systems, including Windows 95/98/NT/2000, Mac OS, Atari, Amiga, BeOS, AIX, Linux, UnixWare, HP-UX, MSDOS, Solaris, OS/2, FreeBSD, RISC OS, MiNT (Atari) OS, etc. There is even a Java port of HTML Tidy.
Tidy Online: This is an online version of the HTML Tidy validator that you can use to clean up your web pages if you're too lazy to download your own copy.
Web Page Purifier: The Web Page Purifier allows you to check your HTML pages for compliance ("purity") with various HTML standards (eg HTML 4.0 Transitional, HTML 4.0 Strict, WebTV 1.1, HTML 3.2, etc). There's a form where you can enter the URL of the page to check, or you can also download the Perl CGI source code.
Web Page Backward Compatibility Viewer: If you need to check your web pages to see if they are compatible with browsers that lack certain features, this page checker might be useful. You actually have to specify which features you do not want on a page, like tables, images, etc. It is probably not as useful as the title of the site sounds, since if you knew which features you needed to avoid, you would not put them in a page in the first place. (It would be better if there were simple options where you could simply select the browser versions you want to check compatibility with, instead of having to specify features.)

Free Web Page Accessibility Checkers

Cynthia Says Portal: Cynthia checks your site against the US Section 508 standards and the Web Content Acessibility Guidelines (WCAG). It only allows you to test one page at a time.
A-Prompt: A-Prompt is a Windows program that checks your pages for accessibility issues and fixes the pages for you (with your help). It has a Wizard-like interface which guides you through the individual (potential) problems that your web page may have and prompts you for information for repairing the page.
Bobby HTML Accessibility Checker: Bobby checks your HTML pages for areas that might pose difficulties for people with disabilities. You simply enter the URL of your web page and it will give you a report after it visits that URL. You can also download Bobby to use on your machine (it's a Java application). It can check your site against the US Section 508 Guidelines, or against the Web Content Accessibility Guidelines 1.0.

Free Cascading Style Sheets (CSS) Validators

W3C CSS Validation Service: This is the W3 Consortium's CSS validator for validating Cascading Style Sheets. You can either download it and use it on your own machine, upload your cascading style sheet (CSS) to their online service, or supply your URL for their spider to visit your site. The W3 Consortium is responsible for the various CSS and HTML standards.
CSSCheck: Cascading Style Sheets Lint: This is a validator for Cascading Style Sheets (CSS) that also looks into a few browser compatibility issues with your style sheet.

Free Broken Links Validators

W3C Link Checker: Worried about broken links in your web documents? This online validator from the W3 Consortium is able to recursively check your document for dead links. You simply enter a URL in the form provided, and it will visit your site and check the links.
Xenu Link Sleuth: Find Broken Links: Xenu is a utility for Windows that checks your web site for broken links. It can work both with a "live" website as well as on a copy of your web site residing on your own hard disk. It's a favourite of many webmasters for checking broken links on their site.