WP-Validate

This plugin adds client-side validation to the Wordpress comment form, using the jQuery validation plugin.

Prevent URLS & Emails in gravity forms textarea fields.

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

Instant, inline validation for Gravity Forms. Users see errors while filling the form, including Repeater fields and custom regular expressions.

Added mail validation function to Contact Form 7. Protected by rejection filter, RFC filter, and DNS filter.

The Validated Field add-on for Advanced Custom Fields provides input masking and server-side validation of other field types.