We just completed and made freely* available two new extensions which integrate two famous contact form plugins: Ninja Forms and WP Forms.
Update: there was a problem on version 1.0.0 of Ninja Forms Integration, just update to version 1.0.1.
Both the integrations work with the free version of those plugins.
You can try them installing the extension you need from the extensions panel directly in your blog admin side:
Integration is rather easy and works in the same way as with Contact Form 7: you can see in the extension the list of your forms and set the mapping between the form fields and the subscription fields. The opt-in checkbox has to be added directly in the form (please follow our instructions!).
*for a limited period of time