ConvertPro specializes in popup boxes and widgets to collect email addresses, and they do that extremely well. They feature a simple drag-and-drop builder and editor. They also have multiple triggers available, page targeting, device detection, visitor detection, as well as full A/B testing capabilities. ConvertPro also integrates with just about every marketing platform and autoresponder out there. Lastly, the have a very reasonable yearly fee, and even feature a great lifetime license option!