Allan Collins

Is your website up to W3C HTML coding standards? This plugin will allow you to test each page or post on your site against the W3C Validator.
Want to contribute? Fork it: https://github.com/allan23/validated

Validated

This plugin will allow you to check your pages/posts HTML against the W3C Validator.

Roland Rust

It functions as a frontend for the Markup Validator Web Service API on http://validator.w3.org. Stylesheets are checked as well.
Languages available: 
+ english 
+ deutsch(german)

Batch Validator

This plugin performs a batch markup validation check over your entire WordPress website.

Validated vs Batch Validator