These are services that offer email opt-in services that are not WordPress plugins. They're services that make it easy for your website visitors to join your mailing list with forms and subscription boxes. They connect to your email marketing platform (like Moosend, ConvertKit, Aweber, etc.) and add your subscriber to your list. You can use these services with any type of website, including WordPress. You’ll need to add some JavaScript code to your website where you want your forms to appear, but that’s easy to do, and you’ll be shown how to do that. At this writing, ConvertBox offers a one-time, lifetime option, while the others are monthly or yearly options. Here are my picks for the three best standalone email opt-in and lead generation form creators.