Complete Website Validation: Free Site Validator
Free Site Validator is a very handy free service for validating webpages.
When compared to W3C validation services, this solution scans not only 1 page but follows links, goes deeper & almost scans all webpages in a website.
The service uses W3C validation service at the backend & enables you to scan multiple websites.
Report generation can take some time considering it scans all webpages but the service can simply notify you when the validation is completed.