The Contact Form 7 plugin is the most versatile and robust form plugin for WordPress. It is also a free, open-source plugin so you can start using it without any cost.
The integration either let's you specify predefined lists or let the users choose the lists that they want to subscribe to when they submit the form.
You can add subscribe checkboxes to new and existing forms. Simply edit the form, choose "Newsletters" from the "Generate Tag" dropdown and insert the subscribe checkbox shortcode/tag into the form for it to display. You can also put multiple subscribe checkboxes for different purposes on a single form.
For installation, activation and usage instructions, please read the Contact Form 7 Subscribers online documentation.
Features of this WordPress Newsletter Plugin Extension
Contact Form 7 Subscribers is an extension plugin of the Newsletter plugin which is easy to install and use to capture subscribers.
Easily generate an email/newsletter subscribe checkbox shortcode tag for your Contact Form 7 plugin forms using the provided interface.
A simple, customised subscribe checkbox will appear on the Contact Form 7 plugin forms which will subscribe the user if it is checked as the form is submitted.