re: Basic forms? Netlify Forms, with JS for just validation. Actual dynamic content? APIs. For example on Firebase, or cloud functions on Azure, AWS, ...
 

Nelify forms have up to 100 free form submissions per month so it might be useful for basic use cases like a contact form.

Unless you need something more to create a emails list you can embed a sign-up form using MailChimp to do that.

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