This is a Popover which displays a dismissible MailChimp subscribe form that triggers after the webpage loads.
It employs:

• HTML5
• CSS3
• jQuery + Ajax

Launch Popover

There are additional options which could implement:

• A timed delay before the popover appears.
• Placeholder text for the field labels within the input boxes.
• Set a cookie to prevent the popover displaying after the user has clicked subscribed or close.