Thanks for posting!
While I am not aware of any 100% free apps for recommended or related products in our app store at the moment, I do think this is a great suggestion!
However, one app that you may like to consider is Personalized Recommendations
This app scales with your business, and is free until you begin to make sales with its services.
Alternatively, what may help you in the mean time is our guide which allows you to custom code this feature to your theme. You can find info on this here.
While themes such as: Minimal, Pop, Simple, or Supply allow you to enable a related products section from the theme editor directly you can also edit this to other themes too via the code.
If you would like to choose the latter, our forum community and experts here may be able to lend a helping hand.