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.
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.
Use Link Checker to find dead links and fix them.
Debugging tools built into the browser.