Dave Doolin

Aweber Super Simple is a plugin that will subscribe a user whenever 
he/she registers to your blog.
The AWeber Super Simple options page allows you to configure the plugin to: 
* Choose whether or not to display the optin checkbox. Users may register if optin isn’t shown, and will not added to autoresponder. 
* Choose a checked or unchecked default status for the optin checkbox on registration. 
* Provide a custom optin greeting message for visitors
This code base is at least once, possibly 3, 3 or more times removed from what was the original 
code base. Wherever or whenever the ultimate source, the base code is old in WordPress years. 
Current effort is to bring everything up to 2.8 coding standards before attempting any extensions.
<h3>Support</h3>
Initial functionality will be supported on a time-available basis.
Installation, configuration and further programming support may be 
available on a contract basis. Contact author with request.

Aweber Super Simple

AWeber Super Simple allows you to subscribe users to an AWeber autoresponder when they register on your blog. NOTE: ALPHA VERSION.  Be careful.

Danny van Kooten

ibericode

<h4>Newsletter Sign-Up</h4>
<blockquote>
This plugin is in maintenance mode, meaning we no longer develop new features or actively support it. Sorry.
</blockquote>
This plugin adds various sign-up methods to your WordPress website, like sign-up checkboxes in your comment form and a sign-up form to show in posts, pages or widget areas.
<blockquote>
Mailchimp user?
Use <a href="https://wordpress.org/plugins/mailchimp-for-wp/">Mailchimp for WordPress</a> instead. It’s free & so much better.
</blockquote>
This plugin works with almost all third-party email marketing services including Mailchimp, CampaignMonitor, ConstantContact, YMLP, Aweber, iContact, PHPList and Feedblitz. With the right configuration settings, you can make this plugin work with any newsletter service around.
Features:
<ul>
<li>Add a “sign-up to our newsletter” checkbox to your comment form or registration form</li>
<li>Easy customizable Newsletter Sign-Up Form Widget</li>
<li>Embed a sign-up form in your posts with a simple shortcode <code>[nsu_form]</code>.</li>
<li>Embed a sign-up form in your template files by calling <code>nsu_form();</code></li>
<li>Use the Mailchimp or YMLP API or any other third-party newsletter service.</li>
<li>Works with most major mailinglist services like Aweber, Constant Contact, iContact, etc.</li>
<li>Compatible with BuddyPress, MultiSite and bbPress.</li>
</ul>
More information
<ul>
<li><a href="https://mc4wp.com/" rel="nofollow ugc">Mailchimp for WordPress plugin</a></li>
<li>More <a href="https://dannyvankooten.com/wordpress-plugins/" rel="nofollow ugc">WordPress plugins</a> by ibericode</li>
</ul>
<h4>Sign-up checkboxes</h4>
Most of your commenters will be interested in your newsletter as well. This plugin makes it extremely easie for them to subscribe to your mailinglist. With the sign-up checkbox, all they have to do is check it and the plugin will subscribe them to your mailinglist.
You can also add the sign-up checkbox to your WP registration form, your BuddyPress registration form, your MultiSite sign-up forms or your bbPress new topic and new reply forms.
<h4>Sign-up forms</h4>
Easily configure a sign-up form and show it in various places on your website using the sign-up form widget, the <code>[nsu_form]</code> shortcode or the <code>nsu_form()</code> template function.
You can set your own messages and even choose to redirect the visitor to a certain page after signing-up.

Newsletter Sign-Up

Integrate your WordPress site with 3rd-party newsletter services like Aweber and YMLP. Adds various sign-up methods to your site.

Aweber Super Simple vs Newsletter Sign-Up