viperchill

ViperBar adds an attractive bar to your site header, which you can use to increase blog or newsletter subscribers. It includes built in styling, Aweber & Feedburner integration, conversion rate tracking, split-testing, themes, and much more.
<a href="http://www.viperchill.com/" rel="nofollow ugc">ViperChill</a>, a blog on Viral Marketing, is the creator of this plugin. If you want to take your blog to the next level, you should read this post on <a href="http://www.viperchill.com/wordpress-seo/" rel="nofollow ugc">WordPress SEO</a>.
<h3>Donations</h3>
You may make donations at http://www.viperchill.com

ViperBar

ViperBar adds an attractive bar to your site header, which you can use to increase blog or newsletter subscribers.

Md Abul Bashar

Just use Feedburner Email Subscriber service on your website sitebar widget.
Full <a href="http://www.codingbank.com/item/wp-feedburner-email-subscriber-wordpress-plugin/" rel="nofollow ugc">Documentations</a>
You can watch this videe 
https://www.youtube.com/watch?v=lqQnR_21UvQ

WP Feedburner Email Subscriber

Does the job

Just use Feedburner Email Subscriber service on your website sitebar widget..

ViperBar vs WP Feedburner Email Subscriber