W3C

Any webmaster worth their title already knows about the W3C Markup Validation service. They may even occassionally use the CSS validation also (though the results from that can be depressing).

The new tool on the scene is Unicorn - a tool that incorporates all the relevant W3C test tools into one report. It was announced in July 2010

The LogValidator is a free, open administrator utility you can run on your own site.

It's written in Perl and distributed by CPAN, this means it can be placed on pretty much any platform. Installation instructions for Log Validator are straightforward for any Sysadmin.

Instead of a one-off site scan, this tool is set to run scheduled, regular tests of your sites health, and notify you of problems as they are created, as reports, or by email.

The Web Accessability Initiative (WAI) from the World Wide Web Consortium (W3C) is ground zero for accessibility guidelines and resources on the web. There you will find the current working drafts, recommendations and guides outlining best-practices on the web today.