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.

joshanderton

Upscribe is a tool for sending marketing emails and emails sequences with Upscribe. Use Upscribe’s simple form builder to make beautiful opt-in forms that can be embedded in your WordPress website by simply pasting your form’s URL into the WordPress editor and pressing enter.
<h4>Easy Form Embedding</h4>
Just paste your form URL into the content area of your WordPress page and vwala! The form automatically appears.

Upscribe

Embed Upscribe opt-in forms anywhere on you Wordpress website.

Aweber Super Simple vs Upscribe