Create both simple and complex Jekyll forms that fit into your workflow. Slapform will handle the server-side integration and simply forward the message to you.
Slapform has a generous free plan and plenty of advanced features such as custom redirects, webhooks, Zapier integrations, and you can even accept payments.
Formspree allows you to create a simple HTML form and, whenever your visitors submit the form, Formspree will handle the server-side integration and simply forward the message to you.
You can use your own fields and you get 50 free emails every month.
Using a natively integrated form on your Jekyll site is a much better solution than using something like Google Forms, for example. For one, a natively integrated contact form is much more professional and being able to collect information form your visitors right from your site is a much better experience and will ultimately lead to higher conversions.
Most of these contact form integrations provide spam protection too and, in addition to the entire backend being handled for you, there really is no reason to not take advantage of one of these services for your Jekyll site.