thebluecube

This plugin is intended to be useful more for developers than other types of users.
Have you ever had a client who likes to be able to independently update the content of the emails that your code is sending out programmatically? If so, this plugin might help you make your client happy.
How to use:
When you activate the plugin, it adds a custom post type to your website called “Email Templates”. You can create your templates using this custom post type. Templates can contain placeholders for your code to fill them up with desired values. A placeholder sample would be {USERNAME} or {USER_EMAIL}. Please note, placeholders should not contain lowercase letters.
This plugin creates a global instance of the BlueCube\Email_Template class which is called $bluecube_email_template. Once you have created a template, you can use the sendEmail() method of the instance to send out your emails.
<pre><code>$email_args = array(
 'template_title' => 'your-template-title',
 'to' => $email,
 'variables' => array('username' => 'JohnDoe', 'user_email' => 'test@test.com'),
);
$bluecube_email_template->sendEmail($email_args);
</code></pre>

Blue Cube Email Templates

This plugin provides an email template system for developers to send programmatic emails which their clients can easily update the emails content.

add-ons.org

Yee Addons

YeeMail — Email Template Builder & Customizer

This plugin great.

Make an impression with your customers and represent your brand well by customizing the design and content of your email

Blue Cube Email Templates vs YeeMail — Email Template Builder & Customizer