Jacques Malgrange

This plugin adds a simple widget that allows you to display an email form and a submit button.
<ul>
<li>You have the options to choose to recommend the site or the current page.</li>
<li>The mail subject and content can be changed.</li>
<li>You can limit the number of recommendation sended by user a day.</li>
</ul>
The widget is only displayed if the user is connected.

Recommend by mail widget

Recommend the site or the current page to a friend by mail.

Sparanoid

Adds readability.com widget on your site. A must-have plugin if you’re a Readability publisher. This plugin also works fine with multisite enabled WordPress (aka. WordPress Mu).
More information please visit my <a href="http://sparanoid.com/work/readability/" rel="nofollow ugc">site</a>.

Readability Buttons (readability.com)

Adds readability.com widget on your site.

Recommend by mail widget vs Readability Buttons (readability.com)