Validation is a process of checking documents against formal standards. In addition to yielding maintenance-friendly files, valid markup and code "future proof" files as browsers become standards-compliant.
Validation Resources
Markup Validation Service
The W3C Markup Validation Service checks the markup of HTML documents.
CSS Validation Service
The W3C CSS Validation Service checks your CSS.
HiSoftware Cynthia Says
Cynthia Says check Section 508 or W3C WCAG 1.0 accessibility guidelines.
Link Checker
Use Link Checker to find dead links and fix them.
Web Developer Tools
Toolbar
The Firefox Web Developer Toolbar offers a lots of tools for evaluating file information and structure and for examining pages as they appear with and without CSS, Javascript, images, etc. The toolbar can also be used to evaluate websites for accessibility
Accessibility Testing
Page Inspectors
Debugging tools built into the browser.