Mark Jaquith

This simple plugin inserts the following into the comment form:
<blockquote>
Not <code>USER'S NAME</code>? Click here to clear this info.
</blockquote>
When clicked, the comment cookies will be cleared, and the form will become empty. This only shows up if the user is not logged in (as most comment forms have a “log out” link for them), and if the user has a comment cookie.

Forget User Info

Inserts a link into the comments form that will clear a user's comment info cookie

wpexplorer

DigitalGdn

This plugin automatically modifies YouTube embeds to use the youtube-nocookie.com domain.

No Cookies for YouTube

Modifies YouTube embeds to use the youtube-nocookie.com domain.

Forget User Info vs No Cookies for YouTube