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.
Check the markup of XHTML documents.
Check your CSS.
Check Section 508 or W3C WCAG 1.0 accessibility guidelines.
Check accessiblity compliance. (This tool was formerly known as "Bobby.")
Find dead links and fix them.
Another use for the toolbar.