Mailchimp Top Bar
The name pretty much says it all but with the Mailchimp Top Bar plugin you can add a full width sign-up bar to the top (or bottom) of your site.
You can choose to make the bar “sticky” so it will stick to the top of your visitor’s screen as they scroll up or down on your site. Bars positioned at the bottom are always sticky.
By default, the bar will only ask your visitor for their email address, resulting in extremely high conversion rates. It is however possible to add more fields to the bar, but keep in mind this will decrease effectiveness of the bar & cause the bar to take up more screen space.
Your visitors can choose to collapse the bar which will keep the bar collapsed on consecutive visits, resulting in a very good user-experience for your visitors.
If visitors use the bar to subscribe, it will automatically collapse.